When importing a text file into a Textworks database, make sure it is ANSI/ASCII encoded, not UTF-8 or otherwise.
You can't tell just by looking at the text how the file has been encoded, but a good text editor will tell you, and allow you to save as another encoding format.
Note that Textworks is not going to warn you about encoding incompatibilities specifically. It's just going to drastically misinterpret characters. Lucky for me this happened before import got underway, as the misinterpretation involved a field name match.
As a nota bene, the above shows how misleading it is to refer to any delimited text file as an "ASCII delimited file," as I've heard them referred to before. Unless of course you happen to mean literally ASCII-encoded as opposed to UTF.