As the title suggests the .50 caliber humvee's glass is literally bullet proof against both AP & API from the M60 & SVD respectfully.

I have to assume this is a bug for a few reasons but primarily, the BTR & BRDM are extremely susceptible to the aforementioned bullet types as demonstrated in the video I have linked below of me in GM with bullet trajectory enabled testing the efficacy of AP VS Glass & APC's.

I forgot to include this test in my video, but for the record it takes roughly 15 KVPT API-T bullets fired from a BTR to fully break the bullet proof glass BUT interestingly enough when I shot the API-T just below the windshield wiper blade 1-3 bullets had pierced through although when they did they lost a visually noticeable amount of velocity, below I attached screenshots of my testing with the BTR's KPVT.

https://www.youtube.com/watch?v=OLwWUX1QRrw (my proof of testing of the AP capabilities.)