I’ve only seen it happen twice, but both times it happened the same way.
It seems if you light one of the gems on the same frame they all turn grey, the wheel will spin again, giving you a second bonus and then turn the only lit gem to grey when it’s finished as if it were a normal spin.
It’s common enough that I’m confident many people have seen this, just not reported it.
Best I can do for evidence is stringing some screenshots together, below you’ll see the spinner is changing positions while white smoke is still spawning coins up top and, final image is of white smoke spawning gems below.
In the last image you’ll also notice that the red gem turns grey while the blue gem stays lit, because this time I triggered it with the red gem and the blue gem came afterwards.