An MSDS on Acrylic shows potential for flare ups…
Hazardous Decomposition Products: In case of thermal decomposition, combustible vapors are formed, which are irritating to eyes and respiratory system, mainly consisting of: methyl methacrylate.
I had no idea what methyl methacrylate was so another MSDS search shows it as a liquid used for making of plastics and it has an amazingly low flash point.
(Not sure these numbers are valid when it is a vapor, but I would think that, as a rule, a vapor flashes easier than a liquid form).
Flash Points: CLOSED CUP: 13°C (55.4°F). OPEN CUP: 19°C (66.2°F)
Easy to connect the dots. As the plastic melts//burns it is gassing a potentially flame producing byproduct.
Interesting, but overall this looks//appears harmless since no danger of it becoming concentrated with the fans running.
An added plus. Since the methyl methacrylate vapor is irritating, then having it burn off is probably a good thing.