When spawning undead mob types on the bedrock blocks of the return end portal, occasionally spawned mobs will be set on fire as though they are exposed to sunlight.

Steps to replicate:

1. Generate a creative new world
2. /setblock ~ ~ ~ minecraft:end_portal
3. Fly to the bedrock at the return portal
4. /give @p minecraft:drowned_spawn_egg
5. Spam-click the corner of a bedrock block that is part of the frame and eventually a mob will spawn and catch fire

Step 5 is repeatable with other undead mob types (e.g. zombies, skeletons, phantom)

Demonstrated here: https://www.youtube.com/watch?v=LBeDNR36Fac