In snapshots leading up to the release of 1.21.2 (24w35a to be exact), the Turtle swimming animation was randomly changed so that their front fins will rotate along the x axis. It is unknown whether this is a bug or not, as it was not mentioned in any other snapshot changelogs leading up to 1.21.2 or the actual 24w35a changelog itself.
This animation difference is still present in 26.2 and affects the new Baby Turtle swimming animation as well. This breaks parity with the new Baby Turtle swimming animation that was added to Bedrock, since they do not rotate their front fins along the x axis there.
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.