The bug
Baby turtles take (suffocation?) damage when pushed or moving against the ceiling. This is similar to MC-50367, but apparently not exactly the same problem since it looks like their eye level ends below their hitbox.
How to reproduce
- Build an upwards bubble column (soulsand) with a solid block above (e.g. stone) 
- Spawn a baby turtle inside the bubble column 
 → ❌ The turtle takes damage below the ceiling
 
      
      
I rewrote your report to describe the problem a little bit clearer. I hope you are fine with this.