Lets be clear - I do not know the answer to this.

What I have read varies. Some say the door plug was installed and bolted. Some say the door plug was installed but not bolted. Some say the door plug wasn't installed - which I suspect from a lot of prior experience is a linguistic gap between engineers and reporters on installed but not bolted.

I remember reading but can't find at the moment something suggesting that inspecting the door plugs and the bolts was a required Boeing assembly line 'job' (think 'ticket'). But none of that matters if someone, as has been reported, later uninstalled the bolts to fix something else, without creating a record of doing so, because they misunderstood the specific procedure they were undertaking.