I’m not 100 % sure but I think these user@NUMBER services appear when a session for a user is created (when a user logs in, for example) and when checkmk fetches the data in that exact moment, the service is not yet up and running. Something like that.
I’d create a rule to just ignore them:
If you don’t want to ignore these services, then reset the failed state in the appliance like so:
sudo systemctl reset-failed user@999.service
But it may reappear then.