As you probably know, when working with mirrored acrylic often you want to just barely cut through it, the foil can bubble up if you overpower the cut. Sounds like you got that part down, which is great.
Unfortunately, all acrylics can sometimes re-weld like that if you’re doing a bare minimum cut. Plain acrylics are pretty tolerant of a little extra laser power so you can give it some extra juice to avoid the re-weld, but since the mirrored stuff is a bit more sensitive so you have to go a different way.
I’ve had best success by doing a two-pass process with mirrored. I would try a new material test with 2 passes and find settings that work for you. You can go lighter and faster with two passes, delivering enough power to cleanly cut while not overheating things and damaging the mirror foil.
If you’ve not familiar with it, my test method is laid out in #6, here: