Zombies and Zombie Villagers standing in the shade can start burning after relogin.
When this happens the block above the zombie seams to have become transparent. Zombies put in that position will burn. The block above the "zombie-position" must be replace before being treated as solid again.
Doesn't always happen. But if a solid block becomes "transparent" it seems to stay "transparent" even if one logs out and in again.
MC-143489 Zombies and skeletons catch fire inside old structures