To answer your question about 'repeat' problems after re-torquing, there have been occasions of this but I think reoccurrences didn't happen as quickly and as yours. As to why something 'backed off' when it's tab-locked was put down to micro expansion of the shaft.
If you're getting that same rumble from the front right case again I think I'd be gutted but I'd have to have another look.....and buy a new tab washer from AJ Suttons for a few pence........unless it's a water pump bearing.
https://www.ajsutton.co.uk/sparepart...ere-2009?uID=0