The baby sniffer hatched invisible, the entity does exists and still can collide with the player hitbox and blocks, and also can dig for seeds. But even in his adult phase it's still unable to be seen.
The steps to reproduce could be that this only happened to me on one specific world (in other creative and survival worlds this never happened to me), which has the same issue I described in MCPE-172538
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We're tracking this issue in MCPE-167886, so this ticket is being resolved and linked as a duplicate.
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