I copied this from a thread in the EU forums. I am experiencing this same exact issue.
"When you right click, you need to move the mouse slightly for the right click to actually register. It’s extremely annoying since you’re mouseover macros often end up targeting something completely unintentional because the right click doesn’t register until you move the cursor.
Most threads seem to mention that it’s an addon issue, most likely AtlasLoot. I’ve completely removed the Interface, WTF and cache folders from the game, and even reset the Cvars using the command mentioned in this support article ."
This seems to have started after the most recent Shadowlands update. I have tried the fixes above as well as /console rawMouseEnable 0. This also seems to happen only during combat. This does not happen during normal travel. None of the mentioned fixes have corrected this behavior. This has made the game completely unplayable for me.