Why would a Lexan mug be unsuitable for microwave use?

I just purchased a 32 oz. mug from GSI. It’s labeled as being BPA-free (which is a good thing)’ It’s also a very hard plastic.

The general guidelines I’ve read concerning using plastics in microwaves warn against using softer type plastics, like styrofoam,which can leech chemicals into your food because they are partially melting, one of the biggest concerns being BPA. The warnings even include the typical (suppsedly) microwaveable containers that packaged frozen items like Stouffers include. The plastics typically used are somewhat flexible and “softer” than other types of plastics. So those warnings make perfect sense to me.

But this mug is a solid rigid Lexan plastic, not the slightest bit flexible at all. So much so that I wouldn’t be surprised if it shattered if dropped the wrong way. But that being said, the overall mug is solidly built and the plastic is quite thicker than average so I’m sure it can handle average dropping.

Therefore, I was quite surprised to see on the bottom of this mug “DO NOT MICROWAVE”.

Would the same prohibition apply to any type of Lexan (as it does for all Melamine products)?

I’m aware that the optimal products to use for microwave cooking are those made from tempered glass and most ceramics. But I thought hard plastics were OK also.

So, is this company just being overly cautious and litigation-shy by putting that on, or is there inherently something about Lexan making it unusable for microwave use?

Hopefully some scientific Jellies can weigh in here and give me the straight scoop. Thanks.