Hello @Henry Woodman ,
Per my test, I cannot reproduce your issue.
By design, Shared columns are read-only for the documents within the Document Set, and they can be edited only for the Document Set itself. When changes are made to the values of the shared columns for the Document Set, these updates will be synchronized to the shared column values for the documents within the set.
If the Document Set stores many documents/files, it needs take a while to take effect.
Thanks,
Echo Du
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