Due to getting more and more lights, I decided to have an extra bridge - rumours said that arround 50 lights, the Hue Brigde could be slow...
So I learned a few important lessons here:
A Hue light can be in zero or one room, but in many zones.
Each Hue light must have a Room to be "persisted" as belonging to the Bride. Otherwise it will be found on each scan by other Bridges.
The last one gave me some hurdles, as many of my outside lights and a few inside lights was only in zones, so when I started to scan the new (second) Bridge, it did find a lot of lights, but not all the rights ones.
But thinking of the 2 lessons above, to move a light from one brigde to another:
Remove the light from all zones
Remove the light from the room it belongs to.
Switch the Hue App to the second brigde and do a scan.
It should find the light that was deleted from the other bridge.
Make sure the light belongs to a room.