In Mongoid, query cache does not apply itself to system collections. This appears to be a feature of the original QC implementation back in 2013 (ef5df80f7c46b4a334fbc8d80b474cebb213b917).

Assuming this is the agreed behavior, the required work is:
Preventing the driver from caching results of system collections
Adding a note to release notes saying the query cache does not apply to system collections
Adding relevant tests
Adding a similar note to Mongoid query cache documentation and/or release notes
Adding relevant tests to Mongoid