Please, even if you don’t want to use the search, at least please take a look at the first page of current threads… the exact same topic is open with replies on what happened: Endorsement loss for joining a game at last second? Well done! - #10 by Vierrna-2113
TLDR: Endorsements decay over time. It does NOT have to do ANYTHING with your matches. It just gets updated after matches and not inbetween or when you start up your game.