It seems that in the normal jira usage you may end-up having subtasks without parents (don't ask me how but that's the reality).

JIRA will convert them to Bugs on the first edit, UNLESS you are trying to update them using REST and in this case it will fail with this message.

I understood that REST is suppose to work like the normal interface but obviously it doesn't.

This bug affects us because it breaks all our automations scripts, scripts that are supposed to update tickets.