I am using 5.10 and this just started to happen with my prints, and it is driving me crazy. I ran a test cube a month ago and it was perfect, now my 20mm cube is printing at 21.6 on the x & Y and 24.6 on the Z.
I am using a homebrew Delta with HH magnetic rods and balls, a custom effector/hotend combo, BTT SKR1.3, etc. This was modded from an FLSUN QQS Pro
I did just upgrade to Marlin 2.1, moved it, and cleaned up some wiring but can't imagine that could be the issue if all else seems to be working. I am about to tear it down, change my hotend, put in a magnetic removable bed plate, add some bracing etc. so I will report back later after I get it set up re-calibrate everything.