I pick a (string) dimension and: filter
I select “is equal to” and then begin typing in the value box.
The suggested values are old. I know that I have added values; but the “select distinct” doesn’t get freshly re-run.
(For what it’s worth, the LookML is tied to a Postgres View, not an actual table. But I can’t see that would matter. if it does, it is worthy of documentation.)