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.