When I had put a warden spawn egg in a spawner, the little mob in the spawner doesn't appear as a warden and is non-existent unlike other mobs which have theirs in a spawner.
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Avoma
I can also confirm this behavior.
[media][media]You can execute the following command in order to reproduce this.
/setblock ~ ~ ~ minecraft:spawner{SpawnData:{entity:{id:"minecraft:warden"}}}
muzikbike
Closely relates to MC-199429
Erik Broes