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:
This mode applies to the '2016 R2' RealSense SDK, which you were working with in another post on this forum. That is why I recommended looking at this link.
I know that you were trying to do facial recognition with the ROS module of the RealSense SDK For Linux before you started using R2 though. But I do not believe that the ROS module in the Linux SDK has the Registration_Mode function, as the Windows and Linux SDKs are very different products.