I have just installed GIMP 3.0.0 revision 1. (Windows 11)
I have created the action “context-brush-radius-increase-skip-10” for the “Scroll Up” event, but this does change the brush size when scrolling up (same problem in the 2.10. version).
When I change the existing action “tools-size-increase-skip” to 10 for the “Scroll Up (Ctrl+Alt)” event, then the brush size is increasing by increments of 10 when scrolling up.