Engines, Engine Plates, Locked Autostruts etc.
When you merge two vessels in the editor this might happen after seperation during flight:
https://imgur.com/a/6CzwGYi
The solution/workourand is to always open first in the editor the upper/highest stage/part of your vessel then merge the later stages/other vessels under it.(The grandparent autostruts go up and not down).
https://imgur.com/a/1Ne8p2E
Please report this in bugtracker .I made an account but i have no idea how to do it .