If a mob is on top of a cobblestone wall it will fall through.
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If I spawn a mob with a spawn egg (I'v tried villagers and creepers) it will sink into the cobblestone wall

Nevermind wrong ticket

I see, yes, mob eggs do cause this, I can confirm that, thanks for clarifying. This is a duplicate of
MC-88096

ok im sorry for creating a duplicate i searched for it before but could not find it
cannot reproduce (or needs more info)