I get this from time to time, too (also on unrated games). What I think happens is that one of the servers that deals with players between games gets rebooted, so the client doesnt't get connected to the end-of-game screen (which comes from that server). It also means that the immediate in-client update after the game doesn't get done, so you remained at 55.21 in the client.
That doesn't mean the game or its result is lost. It will be taken into account at the next nightly update, which is still happening as some years ago. This can be seen nicely in your case as this was your last rated 2P game until now, and the nigthly update happened afterwards. You are at 55.47 right now, up considerably from your previous 55.21 due to the win vs. someone roughly 7 levels higher rated than you.