It's not Windows 10 - don't speculate like that please - the problem is with Riot's end, there are fix's you can do altering your display settings but they are impractical to do before and then again after every time playing LOL.
I have the same issue... Yet it's only been since this morning (6th Aug) when I downloaded 5.15...
Really kinda disappointed because you get those nubs that say oh it wouldn't bother me, well it would and it does - makes the game very unenjoyable.. Text size is f'ed to say the very lest - (You can look at one of the other posts for the windows scaling fix but it didn't work for me)