Queuing up for another game mode before your current TFT match is done breaks champ select.

Given that you die early in a Teamfight Tactics match and leave, you still have the option to queue up for other game modes such as Summoner's Rift. However, if said Teamfight Tactics match were to end while you were in champ select, the TFT post-match screen forcefully kicks you from the champ select screen and back to the home page, but the game still thinks you're in champ select and thus you still get penalized. I'm not sure what would happen if you were actually already in game and then the match ended but it would probably be problematic.