MPGI - Mixed Page Group Index support - was introduced on R18.5. These sets cannot be processed using the Unload/Reload utility.
The IDMS R18.5 documentation says that unload/reload is not supported for MPGI sets. This applies to both the owner and member areas. Here are some details about this restriction.
UNLOAD/RELOAD works on a segment basis. This was not a problem with mixed page group (MPG) support since there were no physical connections between areas in segments that were defined in different page groups. With MPGI it is possible that you have a physical connection between owners and members in different areas that are defined in different segments that are in different page groups. Only the REORG utility has been modified to work on multiple segments within the same execution of the utility. It doesn't matter if you only want to process the owner or the member area as the other unspecified area will be a dependent area in the process and must be processed to properly handle the set connection between the areas. Thus neither the owner nor the member areas in a multi-page group index can be processed using Unload/Reload utility. REGORG must be used instead.