This is by design. The Wwise SoundEngine supports up to 4 Game-defined auxiliary sends. What happens is that the integration builds a list of all ReverbVolumes at a position, sorts that list by priority, and then sends the first four to the SoundEngine. In your case, you only have two volumes at the same place, so both get set.
If you wish to change this behavior, look for AK_MAX_AUX_PER_OBJ in the code (there should be two places, and both lines are similar). The original line is:
for( int32 Idx = 0; Idx < CurrentAkReverbVolumes.Num() && Idx < AK_MAX_AUX_PER_OBJ; Idx++ )
change it to (for only one volume at a time) :
for( int32 Idx = 0; Idx < CurrentAkReverbVolumes.Num() && Idx < 1; Idx++ )