While slabbing an area around a Pillager Outpost, I noticed that strays that wandered onto the bottom half-slabs were not burning when the sun came up. If you hit them and knock them into the air, they will ignite but shortly go out. They also will ignite when they climb up or down a level, I guess for the same reason (leaving the ground briefly).

Here's a video showing it happening:  https://www.youtube.com/watch?v=JdpUd6ieySA

Edit: I forgot to mention that this was happening on a Realm.