Hmmm.... I'm on a late 2017 MacBook Pro, not that that should really matter.
After playing with it a little more, I can see that if I click on the tab, and only move horizontally along the tab row, the tab starts to move a little, then lets go. Sometimes it moves over a whole tab space, other times it snaps back to where it came from, but it lets go of the mouse on its own. If i move vertically, the tab detaches and is stuck to the mouse until I click.
On the Linux side it doesn't happen, but I'm running over VNC, so not exactly the same scenario.
-Tim