Looker displaying Greek letter phi instead of a text string

[Jeff_Franklin]

The values in this field are all short (4 or 5 character) strings, but in the Explore Looker is displaying all of them with what looks like the letter phi (middle column):

when I attempt to drill down into one of them, it brings up a filter that shows the actual string, but if I do try to use the filter, it throws an error.  Is there a known display issue?  Gemini points to character set encoding, but that seems unlikely.

[JuliaKO · Accepted solution]

Interesting case 🤔 I would also test if COALESCE(your_field, 'Unknown') returns 'Unknown' everywhere and check if the auto-generated query uses joins that might produce null values.
Is type: string explicitly set in the dimension definition? Also, what error message do you get when applying a filter to that field?