If I read this right and you are around 550, and beat a player rated around 500 by a score of 5-0, and your rating went down, it definitely was not because of that match. That match raised your rating slightly because you beat the score expectation.It shouldn’t .. I’m pretty new to Fargo. Only play in a league and there are people in my area that are mid 600s that I beat in tournaments plenty but I’m in the mid 500 Fargo lol I only play one match a week your fargo doesn’t move much. Hell I’ve beaten guys 5-0 with about a 500 and my Fargo went downFargo is a joke like all handicap sysems.
Your rating can change as Fargo becomes more aware of the true speed of your past opponents though, even if you haven't played any new games at all. For example, if in the past you won 9-8 against another 550, but as that person got more games in the system Fargo now says they are only a 520, then your rating will also drop very slightly since your 9-8 win is less impressive against a 520 than it is against a 550.
It could also be that some league or tournament you play in was delayed in getting their/your match results to Fargo (it happens, and sometimes they end up getting so far behind they end up submitting many weeks of results at once). Something along one of those kinds of lines is what happened and it was just a coincidence that it was right around the same time you played that match against that guy, but that match definitely did not lower your score and in fact did the opposite (but it ended up being more than offset by something else that coincidentally happened around the same time).