Routers are forwarding convertToCapped commands to shards by using the executeCommandAgainstDatabasePrimary helper that is always attaching UNSHARDED shard version.

The shard ignores the version when instantiating a DDL coordinator (that does not inherit parameters from the command's operation context), but this is not the case when executing the legacy path that is expecting the right shard version.

There are 2 possible solutions:

• Route by using the executeDDLCoordinatorCommandAgainstDatabasePrimary helper that does not attach unconditionally the UNSHARDED version.
• Wait for SERVER-86309 that will properly unregister collections on downgrade (the real issue is that the FCV was downgraded without the collection getting unregistered from the sharding catalog)