VI in 4.40 - this seems familiar but I didn't find - looks like substrings aren't working in Temp fields in VI 4.40 PU3.
Also, temp fields used to display in the import so you could see what they were, and they no longer do. So you have to assign them to some field in the file so you can verify they are working correctly (or not, if you're trying to use the damn sub-string).
And had an upgrade and an import into AP Invoice entry and it wouldn't work after upgrade. Even though Header not checked as being separate from Lines, in Record tab both were listed as available (but I had neither in use). If you set them up, Header would work but no lines if Header first. After putting them both in, seeing wrong results, I took them back out, essentially putting the import EXACTLY back the way it was, and it started working again.
These are the things that WEAR you out.