Shoot your monks

It's definitely an issue at high level too. When I was new to the game, my party had a florge cleric with an unbelievably high AC, with the defensive fighting style from a feat and often cast should of faith in himself, I think having an AC around 25

I was pretty green and running high level monsters, and I remember him ending up being attacked by a solar with a +15 to hit and something in the encounter offering them reliable advantage and I only realised after the fight that it sucked for the high AC character to not just be damaged every hit, but targeted by the thing they've put effort into instead of something like a weak save.

Edit: If I found myself overlooking this frequently now, because I had too much to juggle. I'd probably talk to the player and give their PC magic armor that lets them either turn any attack that hits (that isn't a crit) into a miss, or use their AC in place of a Dex saving throw roll (practically an auto-success), once per long rest, or perhaps proficiency bonus per long rest uses.

That's a pretty janky fix but it puts the tools to retain balance in their hands, and even if it's slapping a bandaid on it, my entire DM style is Dr Frankenstein building bodies with bandaids and kisses.