I've been searching for months for a fix for this outside of using hscroll and I have yet to find a solution that works other than rebinding crouch to a different button. As far as I know this is a probably with all external mice on mac with the exception of, presumably, Apple's magic mouse (which has the issue of having no physical scroll wheel so there's no way you could be precise with it).

The issue is that since 1.13+, Minecraft updated LWJGL 2.9.2 to LWJGL 3.1.6 build 14 which presumably messed with the scroll detection on mac. There is a system default modifier on holding down shift to convert vertical scroll input to horizontal. Apparently LWJGL or Minecraft as a whole no longer accommodates for that so when you hold shift in game now since 1.13+, Minecraft reads it as a horizontal scroll instead of the vertical scroll it's expecting. So basically long story short, Shift + Scroll on mac modifies the scroll input to a different input the game doesn't recognize and ignores it.

As far as I can tell, Karabiner doesn't have the functionality needed to fix this, as it seems to be missing a way to actually detect a scroll wheel input. So far the only external program I've found that can disable the horizontal scroll was "Mos" but it introduces it's own problems forcing smooth scroll to be on which overrides the discrete scroll function in game meaning 1 scroll click scrolls like, 4-5 inventory slots with it.