My understanding (drawn from this post: How can I make a lookup field show only unique results? - #23 by Mathias_Mayer) is that using automation to act on update of a rollup field (leveraging ARRAYUNIQUE) should work for selecting matching items in a linked items field, as though each string were entered by hand; but it does not work when operating on linked tables. It only works for choice fields in this context.
Trying to operate on field with it set to lookup generates "insufficient permission" errors. The only hint on that I found was here:
“For context, the error you are receiving can happen when the cell value for the linked record is not a valid primary cell value for the linked table (e.g. if the primary column in the linked table is a computed field).”
[ Creating the record failed due to insufficient permissions ]
Which…what? When I type the same string into the field, it finds the record. So why should this make a difference? But it is a calculated field in the source base, and a long text field in the sync’d version.