My guess of what may have happened reading between the lines of many reports is - Boeing originally instructed Spirit to loosely fit the plug and not secure it

My guess of what may have happened reading between the lines of many reports is - Boeing originally instructed Spirit to loosely fit the plug and not secure it because they planned to remove it to fit out the interiors. In this case Boeing would be accountable to secure the plug door after the interior is done to the right specs. Somewhere down the line, Boeing probably decided not to remove the plug to fit the interiors and instead just load them through the main bigger openings only. They probably did not document/log this change in practice and think through implications and probably did not update instructions to Spirit to secure the plug door going forward (or maybe they did but Spirit didn't pay attention) Hence we have many planes with Spirit assuming Boeing will secure it as per the previous practice and Boeing not securing it because they didn't remove it while fitting out. Now the obvious question is if Boeing or Spirit has deviated from documented production practice in this area, have they deviated in other areas and hence the entire production process of max lines will probably need to be re-audited