It’s unlikely in this situation that the bolts were ever tightened correctly the first time.

There are plenty of ways to ensure bolts stay tight that work, and these plug doors have been in service on other planes for a long time with no issues.

The entire rotor assembly (literally what holds it up) on some helicopters is held on by a single nut, for example. [https://en.m.wikipedia.org/wiki/Jesus_nut]

This doesn’t appear to be a case of the engineers not designing it safely - but it not being installed correctly and no one catching it before it rolled out the door.