Thanks for the additional information.
Those people in the chat should report their issues so we can fix them! 😆 Don't suffer in silence! We want to fix issues and appreciate reports.
3.8 is very old and different in many ways, so that it works there doesn't really help us narrow it down.
Since double click works everywhere else, that likely eliminates any stylus drivers or interference from other software. Since double click works sometimes, it's likely something to do with how you are clicking and how we detect and apply the clicking.
After some digging it seems the problem is that moving the mouse at all (1 pixel) when the mouse button is down will start a drag. If a drag has started, then double click won't delete the key. You can see the drag has started when the key under the mouse is cyan rather than white.
We'll fix this in 4.2.39 so the drag doesn't start until the mouse moves a significant difference, like most drags in Spine. Sorry this caused you so much trouble, but thanks for letting us know about the problem!