Shimano 105 rear shifting issues
Assuming that the cabling is clean, that the derailleur is nicely indexed and well alligned, the derailleur hanger not bent, and the chain correctly moves lateraly +/- 4mm for each gear up or down, then I would have a look at the distance between the upper jockey pulley and the sprockets.

The distance should be on all sprockets less than 25 mm and more than 12 mm.
I suspect that your distance is slightly too high, meaning that the chain has too much room and can flex.

The way to adjust this is by loosening the B screw, as described on this page.

I had the same problem on a 105 derailleur on one of my bikes. B screw was loosened to the max, and I was still too far from the sprockets compared to my other bikes. I managed to file the part of the derailleur hanger touching the support of the derailleur where the B-screw is, to shorten this distance even more. Now it shifts very well.