Flags are fixed size, as noted the maximum number of characters allowed is three as noted here. I’ve checked and text in flags scales with zoom level.
The ellipsis you see in your screen grab is because if you exceed the expected/documented maximum three text characters, the label cannot fit the fixed-dimension flag.
So there is no error here—by you or the app. Rather, you are unwittingly assuming a feature that doesn’t exist. My recollection is that characters in flags were never intended for actual words (e.g. ‘read’ or ‘review’) but for codes such as “A1” or “BZ” where the characters and not words imply meaning.
If you have use case for a feature where flags must support words, by all means email firstname.lastname@example.org with your use case and supporting info.
Don’t overlook subtitles as a possible alternate route.
So, sorry your issue is not fixable, even if for reasons that there is nothing to fix. But, I hope that’s helped.