Even after I have mapped the G-99 extra action button to an action bar it still shows up at weird moments all over Undermine. This was also a problem in Azj with the faction bonus button,
Try resetting the WoW User Interface. Otherwise if that doesn’t work, then next best option is to report it as a bug
I ran into this issue myself, and just found a workaround. I suspect the cause was the default UI not understanding that ElvUI was actually displaying the action bar that I put the button on. I was able to “fix” it by going to the blizzard options menu, going to the Gameplay → Action Bars section, and ticking the boxes for Remove Bar 2/3/4/5 Action Page. This didn’t actually change my UI because of ElvUI replacing it, but it did at least trick the default UI into thinking that the G-99 Breakneck button was actually on an action bar.
I was having a similar issue with Bartender4, and it was because some of my action bars weren’t “enabled” in the Blizzard menu (even though I had access to them through the addon)…so the game didn’t recognize that I’d moved the car to a bar.