I have had this a few times when upgrading fleet versions, as i always upgrade with every minor version.
Something happens in the upgrade / migration of pipelines that an Integration is upgraded and the pipeline for the latest version is not there. Happens more when you just upgrade just an Integration and not the Fleet stack whilst upgrading the ELK stack.
The only way i have found around this was to totally remove every instance of the Fleet Server Policy and it's agent and start with a fresh Fleet Server & Agents install, totally removing all data, files and folders.
There is certainly a problem that comes with upgrading existing fleet setups.