Weird acrylic is sensitive to being overpowered. It’s possible that this batch was slightly different from previous batches until your old settings won’t work the same way. I would suggest doing a material Tess on each batch that you get to verify what settings work best for that particular piece of acrylic.
If you’re not familiar with running material tests it’s a very simple process and I’ve outlined it in item number six here:
Lol, I’m glad you were able to make sense of that. I dictated that response to my phone and the translation was pretty good but not perfect. Mirrored acrylic is sensitive to being overpowered. There are many posts about problems like yours where the mirrored foil gets damaged along the cut lines. Do you want to find the speed in power that just barely cut through your acrylic without applying too much power. Some people have experimented with doing multiple lower powered passes and I’ve gotten better results.
Try cleaning your crumb tray. Acrylic is sensitive to all the little things, including changes in the humidity of where you live! I noticed I was getting a lot of redeposit on the back of the acrylic mirror that I was cutting, and cleaning the crumb tray really helped with an overall cleaner finished item.
I also agree to make sure your crumb tray is clean. When I have residue or fallen scraps in the crumb tray, I feel like they start burning and affect my material. When my crumb try is very clean, flashback is very minimal.
I hear all these tips and they’re good, but I think they may be dead ends here.
First, this is not a flashback problem, this is heat damage to the mirror foil, which is on the first surface (top) of your mirror. Flashback happens on the bottom of your material in a pattern that matches your crumb tray hole spacing, and that’s not happening. Therefore flashback mitigation, while a good idea, is not the solution to this problem.
Secondly, humidity may affect cutting some materials, and acrylic is somewhat able to absorb water. However, even if the issue were humidity in some way it would lead to laser power loss, and you’re not seeing that. Humidity is not your problem.
Lastly, ambient temperature does affect laser performance. The Glowforge will tell you if it’s too hot or too cold; if you’re not getting those errors then that’s not a likely concern. Besides, if you’re out of temperature range the laser power will reduce, and you’re seeing evidence of overpowering. As with humidity, temperature is not a likely problem here.
That leaves power and variability of your materials as the far most likely issue. I still think my initial advice is the right track.