In snapshots leading up to the release of 1.21.2 (24w35a to be exact), Turtle swimming animations were randomly changed so that their front fins will also rotate along the x axis. This is possibly a bug, as it was not mentioned in any other snapshot changelogs leading up to 1.21.2 or the actual 24w35a changelog itself.
Step to Reproduce:
1. Spawn a Turtle in the water on 1.21.1 or an older version
2. Spawn a Turtle in the water on 1.21.2 or a newer version
3. Look at the Turtle's front fins while it is swimming in both versions, they will rotate along the x axis after 1.21.2 despite this change not being listed in any changelogs
Expected Result:
Turtles only rotate their front fins along the z axis while swimming like they did in previous versions OR this change is acknowledged as intentional
Observed Result:
Turtles will now rotate their front fins along the x axis while swimming
I am able to reproduce the reported behavior. It was introduced in 24w35a.