Composite columns are treated as "text, shown in tag browser" columns, assuming the 'show in tag browser' box is checked. There is no way to get them to be treated as "text, tags-like", columns, internally known as "is_multiple" columns.
The only workaround is to use search/replace (edit metadata bulk) to copy the value of the composite column to an is_multiple column (tags or some custom column).
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