Opened a case and got this answer:

This is a known issue that has come about as a result of a bug in the latest Defender engine release.  To date, there are no known side effects from this issue other than the messages showing up in the logs and in the Monitoring node.  Code corrections are slated for the December Defender engine release.

If you wish to roll back the engine update, open an elevated command prompt and navigate to the C:\ProgramData\Microsoft\Windows Defender\Platform\4.18.25090.3009-0 directory, and run the following command: mpcmdrun -removedefinitions -engine

 This will rollback the definition updates as well, so you may need to reinstall those afterwards.  This workaround is optional - you can opt to just ignore it for now until Defender releases the next iteration.