This issue may be functionally related to #1 from this post (#ODNT-9817), but presents differently.

System

• Windows 11 24H2 (26100.3775) Pro
• Start11 v2.52.3
• 2 Monitors
  • Primary (1):  3840 x 1600 (On right)
  • Secondary (2): 3840 x 2160 (On left)
• RTX 5080 GPU
• Enhanced Taskbar
• Taskbar on top
• Taskbar (Start11) Auto-hide
• Start Menu left aligned
• Custom Start Button

When the taskbar is on the bottom of the screen, desktop icon grid snapping is as expected, in that you need to move an icon roughly the same distance up or down to get it to move the next spot above or below its current position. There is a slight bias towards moving the icon down, and I haven't tested the distances with the vanilla taskbar (i.e. Start11 disabled) to see if the distances are exactly equidistant there, but with the Start11 taskbar on the bottom the movement thresholds feel more-or-less correct (though just be advised that there could be a slight discrepancy compared to stock Windows, even when the taskbar is on the bottom, since again I haven't compared directly).

The issue, is that when the taskbar is switched to the top, for whatever reason the thresholds for snapping to the next spot in the grid completely change and become massively biased towards an icon moving up; that is, the icon has to be moved ridiculously far down for it to go to the lower spot, and only a smidge upwards for it to move to the higher spot. 

I tried to tease out the exact point past where the icon would actually move for each position of the taskbar as best I could in order to create the following visual that summarizes the problem.

This is at 100% desktop scaling and of course the pixel counts vary depending on monitor size and whatever icon size I'm using, but its just to get the point across.