So, your problem is that the Robust Metadata Adapter does not instruct the replacing Remote Metadata Adapter on how to recover the current state of the user sessions.
We confirm that there is such limitation in the Robust Metadata Adapter, which, in fact, assumes that the Remote Metadata Adapters persist the current state.

The Robust Metadata Adapter only guarantees that if a Remote Metadata Adapter answers to the notifications after persisting the information, then the newer Adapter will receive all the notifications occurred in the downtime that will not have been persisted (with a few possible duplicates). Note that this only includes session and subscription closings, because other requests must have been refused during the downtime.

The above, however, leans on the assumption that the new instance of the Remote Metadata Adapter is able to know when it is in replacement of a previous instance.
Could you manage this information?
Admittedly, the Robust Metadata Adapter does not help in this regard at the moment.