While I was messing around with baby sniffers, I noticed that when I summon one in, their head appears to be smaller on the first frame of their existence. The very next frame, the baby sniffer's head pops back to its normal proportions. This is a very weird behavior, and you can replicate it using the following spawn egg command I used to first discover this bug occuring.

/give @p sniffer_spawn_egg{display:{Name:'

'},EntityTag:{id:"minecraft:sniffer",CustomName:"",Age:-100000000}}

*Just to note, I would include a screenshot of the frame of the sniffer's head being smaller, but I am not good at timing both spawning the baby sniffer and taking a screenshot.