Next time try annealing first (170F iven for 1hr/mm thickness). Or IPA. I’ve had IPA work fine - might be the water in it helps prevent the full on reaction you saw from denatured.
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You’re right, it is the heat stresses, and any solvent is enough to kick off the crazing and cracking.
Annealing it like @jamesdhatch mentioned will definitely help. TAP plastics has good info on their website and youtube about working with acrylic.
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I know alcohol in general is not to be used on acrylic becasue it will cause crazing to some degree, even if it’s too small for our eye to detect. (Something about it being a mild solvent, polymer chains getting aligned or some other scientific stuff I don’t know.) I have to admit that I used it numerous times and never experienced any large cracks and it’s generally not been an issue. Then I was working on a complicated acrylic puzzle and during each stage of the build I had wiped it down with alcohol. When I finished, the piece was destroyed by the tiny crazing which was especially noticeable in the clear pieces. (OK, not really destroyed, but it looked like crap. ) I keep it sitting on the table to remind myself not to use alcohol lol.
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Thanks, all.
Yes, I’ve found that even 70% ipa produces some cracking, and I can take the risk.
I’m going to try cutting with the work piece elevated off the tray with small spacers, to see if the separation by a couple of millimetres of air is enough to avoid the issue.
If that doesn’t give me a smoke free cut, I may have to just saw the stuff out !
John 
Good news folks !!!
I’ve just done a simple test cut, a curved line., through some goofgrade 1/8" acrylic from Home Depot.
I first ‘cut’ the line on a piece of mount board, to give me a location, then place three magnets in appropriate positions to avoid the beam.
I then placed the acrylic on top of the magnets, so I now have the plastic, with an air gap of about 1/8" below it, over the test mark.
Fired up, making the necessary adjustments to the thickness, but using the 125/full settings that I previously used for this sample of material.
No ‘hazing’ around the completely clean cut, with only a small amount at the beginning of the line. Not sure why, but the result for me is a perfect solution.
What I’m doing is to cut acrylic glazing for my small fan leaf display frames , and now a very happy bunny !
John 
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Yup, learned that one early on with cnc engraved acrylic. Use windex or just soapy water.
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FYI, isopropyl alcohol is a major component of Windex. 
Not saying stop using it, just keep it in mind.
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Noted, and thanks for the observation.
I did a full size test, but standing above a piece of paper to mke the location mark.
Promptly set the paper ablaze, so still finished up with some hazing.
Getting close to a solution though, and I’m hoping to finish up not needing any cleaning at all, so no problem.
John
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We use the vinegar version. Maybe it doesn’t have that
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Plenty of science lab people have learned this the hard way as well… we had to replace the whole front of a contained workspace when someone didn’t read the sign about (im)proper cleaning agents. 
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I’m not sure if I should be embarrassed by this admission, but talking it over with both SWBMO and a friend (antique trade) re the use of acrylic vs. glass in my frames, I’ve decided I need a plan B.
So I’ve just given myself tommorrow’s birthday present to myself - a Taurus 3 ring saw !
For those not familiar with this beast( and I count myself as one of them), it’s a diamond dust coated steel wire ring that will cut my glass in all directions. This gets over the major problem for me, which is to cut a concave curve, about 3" - 4" radius, out of the bottom edge of the frame glazing.
So now, with Glowfinger and the yet-to-be named glass cutter, I can steam ahead. This was the last major problem to be solved.
John
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Want to understand here - the cracking from cleaning with alcohol only happens when the acrylic is in-contact with the tray while cutting?
Raising even a mm or so is enough to prevent that?
I’m working on several acrylic pieces so soaking up info as I go…
I think the process is this. If the acrylic is laying on the tray, and this would be true even with an applied masking on the rear, I think, the extra energy from the laser, reflected by the metal tray causes small ‘pits’ that can be seen along the bottom edge.
This seems to be a point where a thermal stress gets locked into the edge.
[It’s just occurred that if you had a couple of sheets of polarising film, you should be able to show this as tight coloured bands around each pit.]
When you apply even gentle solvents, like ipa, they will attack these pre-stressed points, and initiate cracking.
When I lifted the acrylic on spacers 1/8 -1/4" higher off the tray, I had no sign of the pits, and no effect by solvents. What you then have to allow for is to support both sides of the cut, to prevent the waste piece drooping off, and moving the work.
There will be a minimum separation, but I guess 3 -4mm should be enough, 6mm being safe.
Hope that gives you a handle on it.
John
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So have we concluded that the best way to clean masking adhesive residue and smoke residue from acrylic is soapy water or vinegar?
I’m resistant to adding more steps to my process to raise each piece and properly support all pieces from dropping or distorting as cut, but interesting idea.
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I use Novus plastic cleaners to remove the residue.
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Any particular one ?
Walmart seem to have several different ones - polish/scratch remover etc.
John
I usually use Novus #2 first followed by Novus #1. I use a microfiber cloth.
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Second vote for Novus#1. When I worked in Tradeshows we never used windex on the acrylic displays, it causes irreversible hazing. Novus has a bonus anti static effect to keep dust away. 
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