I like to keep variable groups in alphabetical order. When I drag and drop a group to reorder, it works about 20% of the time. The other 80% of the time it either drops the group into another group, or just ungroups the group entirely.
Am I missing something or is this interface just broken?
I understand your frustration. There's a known behavior where dragging a group onto a group that already has children nests it instead of placing it above or below. The drop indicator is the tell: a black line means it'll land between two items, a blue highlight means it'll go inside as a child. If you're seeing the blue highlight, nudging the cursor slightly toward the left edge of the row usually flips it back to the line. A couple of quick questions, so we can investigate your specific case:
Is this happening in the desktop app or browser, and which OS?
Are the groups you're dragging onto ones that already contain nested groups, or does it happen with non-nested groups too?
Is this a local collection or one from a subscribed library?
With this info, I can help connect you with our support team to take a closer look.