Ex-aircraft mechanic here. Nothing will have been done in this situation without paperwork backing the decision. There are often small niggles that could ground an aircraft, but there are manuals that can be consulted to see how many more flights can be taken before it must be grounded for rectification - the MEL (minimum equipment list) and CDL (configuration deviation list). So the airline will not have made the ultimate decision to keep flying, Boeing will.
The fact that this has now been found in two different airlines means that it’s a design flaw again, either the locking mechanism on the bolts is insufficient, or the reinstallation instructions in the maintenance manual is incorrect (the Alaska airlines aircraft door plug was recently removed to carry out maintenance on another part)
It’s a door plug, which means it’s meant to be replaced with an actual door if required, so a lot of the hardware for an actual door are in place. Doors are designed to slide in, then raise up so the stop pins engage the stop fittings from the inside, so the door is in effect bigger than the hole it’s in. this video provides a detailed explanation of how it works.
The big issue here is that the airplane is only 2 months old, it was delivered from Boeing in late October. Which means it’s either a design flaw or a process flaw in the original manufacturing. This smacks of corporate cost cutting again. Boeing are totally on the hook for this and it’s only lucky there were no lives lost. You watch, they’ll blame it on the airline initially but the fault will come back round to them again.