I like how Riot is completely ignoring this problem, and find it pretty sad that Riot doing others things than please their players?
You guys got to realize something..there are A LOT of discussions being posted every minute on top of all those posts getting comments every minute. Riot needs to validate problems and there is a process to that. Also, if this is an existing problem which it is, they are most likely are not going to post again and again because it is your responsibility to research the problem before just posting about it. This allows new emerging problems to be fixed while if older problems do pop up and number of complains reaches "X" they investigate it to fix it. Matches not showing up has been here for a long time..here is couple links
http://boards.na.leagueoflegends.com/en/c/bug-report/aBGifKRy-ranked-game-not-showing-in-client-match-history-but-it-shows-online?show=flat&page=3 (2 month old)
http://boards.na.leagueoflegends.com/en/c/bug-report/fnVE6QnW-match-completely-missing-from-my-match-history
( 1 month old)
if you look at the first link..you will get your match. just wait for it. it can take up to a couple of days but you will get it. and if it was a case where riot was having some sort of problem, say everyone was experiencing lag and all, you will get loss prevented if you lost and win if you won.