This might well be due to moving a comp or comp-3 field to a screen variable and trying to send it. The problem is that, especially with everyone using an "emulated" 3270 terminal, it seems that the "emulated" terminal generates some strange PROG### numbers, so that it is difficult to match those PROG###'s to a specific problem.
If your screen is doing strange things ( and sometimes things that you thought impossible for a screen to do, then suspect the above problem. I think that the underlying problem is that the comp or comp-3 are interpreted as field attribute bytes and that can really result in some problems.