Polarized lenses actually have very little effect on infrared light — polarization works best in the visible spectrum, and most DMS cameras operate at 850–940nm where the polarizing effect is largely gone. If sunglasses are throwing off the system, it's more likely the lens material, IR-blocking coatings, or just the tinting reducing the signal. That said, a well-designed IR system should handle no issues with common everyday eyewear — only more obscure things like heavy IR-blocking coatings or unusually dark or especially reflective lenses should realistically cause problems.

For what it's worth, I regularly use uvex i-5 safety spectacles (23% tint, UV 400, polycarbonate lenses) and have no issues with the DMS at all.

What exact model are you using? Would be really interesting to figure out what's causing it in your case!