So I got some strange shifting issues with my 105 road shifters (ST-R7020) combined with GRX derailleur (RD-RX812). The cassette is deore CS-M5100-11
Now in theory, and according to Shimano's compatibility chart this should be compatible configuration, despite a little mix-and-match
https://productinfo.shimano.com/en/compatibility/C-454
Now the problems I have screams still for compatibility issues with slightly wrong pull ratios
Going from smallest cog to largest, the first 4 cogs are shifting perfectly. Then the next couple clicks want to over shift and skip cogs, and the largest 3 are shifting perfectly again. In total it takes 9 clicks to go through the 11sp cassette. What fucks my mind over, is the top and bottom cogs that shift perfectly, if the pull ratio was wrong, these would have equal problems?
I'm also running a Frankenstein's Monster drivetrain: 105 shifters, Deore XT rear, Deore (XT? can't recall at the moment) 11-48 cassette. To make things even dicier, I'm using a Wolftooth Roadlink to fit that cassette. When I first set up this drivetrain, it worked flawlessly (shockingly :D). I share that context as empathy for your shifting woes, and I apologize in advance if the following is obvious/already been tried.
Quite suddenly, rear shifter started exhibiting the exact behavior your describe; I suspect someone messed with my barrel adjuster as a prank because all of my riding group suddenly had shifting issues after that bar stop. I couldn't get the shifting solid again until I loosened the pinch bolt and performed the cable pull adjustment as if it was first install. And with my sketchy setup, I have to follow the steps very strictly.
This Park Tools video (https://www.youtube.com/watch?v=UkZxPIZ1ngY) will be old hat for you, but I include it for anyone else trying to dial in their rear derailleur.