/u/RiotTuxedo, is this something Riot is aware of?
I remember noticing some delay in updating player's list of matches, but I wasn't able to quantify that delay. Haven't heard anything from community developers on this issue apart from "this match is missing" and then later when I check for the match it's there.
If this is something players are noticing I can definitely bring it up to the team. We're mostly focused trying to ensure we transition as many third party projects over to new match history and that we can support their traffic. I'd say that's still our priority but my hunch is that is likely due to long cache times and cache evictions not working as expected.
TL;DR Had a feeling, wasn't sure how big or widespread this might be. If players are noticing it, I can definitely escalate it.