I remembered that there were some similar cases to yours back when RealSense first launched in 2014, so went back in the older RealSense forum to research it (this forum is the second, newer RealSense forum).
RealSense expert Samontab thought that the IDs were being incremented because the user was being unregistered, and so a new ID was being created when that same person was registered again.
This discussion on the old forum may be relevant to your problem.
When someone else had a problem with retaining a registered face, Intel support member Colleen Culbertson suggested changing registration from Continuous to On Demand mode. I.e change
In other words: