This happens to me ALL the time nowadays, and there are two different scenarios. I haven't noticed any particular course of actions that cause it, as far as I can tell it's random.

1. Characters are just flat-out missing from the screen. The game believes they don't exist, and the list is short (sometimes very very short, sometimes only 10-20 missing).

2. The UI obviously understands that the characters exist, but their node is missing. IE: Blitzcrank BLANK Caitlyn. You can't click it, and there is no image, just a blank spot where the champ picture/name should be.

Nothing seems to fix this except for waiting for a few minutes (I'm assuming so that some sort of memory purging happens), restarting, or playing a game.

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I get that constantly, this needs to be fixed.