I just don't use tab grouping (yet) and I don't want this particular auto-group feature which I keep triggering by accident
Found this annoying too but didn't think to do anything about it until I read this post...
Anyway looking it up, maybe try
about:config > tabs.group > browser.tabs.group.dragOverThresholdPercent
I guess turn it to 0?
Edit: nevermind this isnt an option anymore on my end.
Could also disable via browser.tabs.groups.enabled