The Axolotl tail is can go through blocks. The same happens with the player's legs while on a minecart.
Environment
Windows 7, Java Version 1.8.0_151
MC-209086
Axolotl Tail
Resolved
Duplicate
0
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.