During the upgrade to FCV 9.0, every timeseries collection is implicitly converted from the legacy (viewful) format to the viewless format. This conversion breaks the counters reported in the catalogStats section of the serverStatus command:

• timeseries drops to 0, while it should remain unchanged (the collections still exist, they are just converted to the viewless format).
• internalCollections should decrease by the number of timeseries collections, since the conversion removes the legacy system.buckets collections; instead it stays unchanged.

The in-memory CollectionCatalog stats are only updated via collection register/deregister bookkeeping: a collection is counted towards userTimeseries only when it is already marked as a viewless timeseries collection, and system.buckets namespaces are counted as internal. The upgrade to viewless format performs a dropView and a renameCollection of the buckets namespace over the main namespace without updating these counters, leaving them stale.

Restarting the node fixes the counters, as the catalog is rebuilt from the durable catalog on startup.

Related: SERVER-106489