yeah that stupid bugs still in the game, its sad really. My computer has been able to run any and every game I throw at it, and the only ones that cause me as much trouble as league are my super heavily modded games, which is kinda expected when you have that many mods, and the game I've been helping my friend test which isn't really even in alpha yet as its only me and him playing it, once again expected... Its kinda sad just how many issues are in league and how long some of them have been there. I gotta say the biggest offender for me is the play again > custom lobby bug in ARAMs. That things been in the game almost since the ARAM game mode was released. I clicked play again riot I don't want to play customs, I wanna play ARAM with my friends like we have been for the past 2 hours.
Back to the bug you're having issues with based on the experimentation I've done so far, so long as the league client is the active window when the loading screen ends you're safe, of course thats just annoying AF when you have multiple monitors and are picking a playlist, or quickly sending a message in skype, or any other reason you have to alt tab out.