I noticed some zombies were in the sunlight and not getting burned and I spawned a few in full sunlight, no shade available, and some of them weren't getting burned at all, even if I punched it. Some zombies did burn though but there was few who were perfectly fine. I took some screenshots of my little experiment and they're attached with this report if that helps.
Update: I just went to kill them and it turns out the armor they were wearing isn't visible so I changed my report title. So basically the armor is invisible. 😅