Cannot see dataset partition properties with SSMS any more

Until very recently I had no problem checking my incremental refresh dataset's partitions via SSMS.  Now the attempt to right click on the table and select "Partitions" returns an error: 

Microsoft SQL Server Management Studio
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The given key was not present in the dictionary. (mscorlib)

SSMS version is 18.5.

Still works fine with DAX Studio (and also works in SSMS when running the MDX query

select * from $SYSTEM.TMSCHEMA_PARTITIONS

), so I am a bit at a loss what might cause this or where to look?