I have tried your report with my sample data, and using your formula for the description, I have no problems. Since my sample data is likely different than the data you are using, perhaps proper information is not being obtained due to the links in your report.

I would ensure that you are linking 1st from OELINE to OEHEAD on Prefixed Doc #, then from OEHEAD to ARCUST on Cust #.

If this does not help, you could then add another line to your formula:
If Line type = "REGULAR_LINE" and Line number = 1 then Description else " ".

By only looking for "Regular lines" in line 1, the report will focus on actual item lines entered, rather than comment or other misc line items.

Hope this helps.