The Axolotl tail is can go through blocks. The same happens with the player's legs while on a minecart.
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Avoma
MC-209086
Axolotl Tail
Resolved
Duplicate
0
Avoma
iv
(Unassigned)
Unconfirmed
(Unassigned)
20w51a
Retrieved
Hey there!
The legs going through a minecart is a duplicate of MC-150016 and with the axolotl tail, I believe this is normal. This same idea happens with other entitles as well, including guardians, elder guardians, and even the player. For example, if you walk really close up to a wall, your arm will actually go through it.