@efpbe, Are you sure that is the 'vSAN Default Storage Policy' applied to that object?(I think it is possible to still have multiple of these in SPBM inventory)

Asking as that object does not have 'No preference' rule on it - you should change it to 'No preference', apply it to all objects and validate this (from 'esxli vsan debug object list --all' output) has been applied to everything (not just what is registered in inventory!) that has 'Dedup&Compression' rule, then you should be able to reformat to Compression only (and change policy to that also, but not mandatory).