The billboarding effect used to render fire doesn't work correctly for rendering mobs inside of a spawner; it's rotated incorrectly and thus the billboard effect fails. This seems to be due to the billboarding not accounting for the rotation of the entity in the spawner.

To reproduce

Create a mob spawner with a flaming zombie by running

/setblock ~ ~ ~ minecraft:spawner{SpawnData:{id:"minecraft:zombie",Fire:12s}}

Then, switch to spectator mode (so that the entity doesn't rotate) and look at the entity. It will sometimes not be facing towards the camera. (It seems to face towards the camera properly when the entity inside is looking in the +Z direction, while it doesn't render at all when it's in the -Z direction and renders skewed when it's slightly +Z with significant X values). To rotate the entity, press F3+N twice to quickly switch out of and back into spectator mode.

This is also visible in other modes, where you can see the fire rotating with the entity instead of facing the camera; however it is much harder to notice the incorrect skewing when it is rotating and the flames from the spawner make it hard to see.