If you have prerun-time arrays that overlap in data structures, the order of loading these arrays at run time may be different in RPG III and in RPG IV. This difference in order can cause the data in the overlapping section to differ. The order in which the arrays are loaded is the order in which they are encountered in the source. This order may have changed when the arrays were been merged with the subfields during conversion.
In general, you should avoid situations where an application consists of OPM and ILE programs that are split across the OPM default activation group and a named activation group. When spilt across these two activation groups, you are mixing OPM behavior with ILE behavior and your results may be hard to predict. Refer to Program Creation Strategies or ILE Concepts for further information.