When holding a baby horse on the lead it goes through blocks and dies.

Small horses carried on leads can not jump over blocks. This makes them go through the block and suffocate and die. This happens in both creative and survival. This happen sometimes.
To find this bug:
1. Get a small horse and tie it to a lead.
2. Try to go up a small hill. You must walk for a little and it will start affect.
3. It starts to suffocate.