Something similar like this happens to me. Ive realized that something with sivir is causing game crashes. Or causes the game to completely not load period. When i get sivir in urf (all other champs fine) my game gets stuck at 60% load and stays that way until the end of the match. Closing the client causes the game to make me disconnect from the match and it never registers that i ever reconnect because when i close the game out i then get stuck at 8% load screen. Currently stuck at 4% and nothing is happening. Im avoiding sivir from now on until riot actually resolves this issue
edit: so ive been able to team up/go against sivir. but this match im currently in has a sivir and the same thing happened. now I believe that it is the pizza delivery sivir skin that's causing this issue (for me anyways)