In the Configuration System script, under the Individual Spritesheet Center
there is a line like this:
MNK_POSES_ACTOR = {1 => 18,2 =>12,3 =>16}
Ensure the Actor ID of your second battler has the correct number of Poses (or rows) specified for its sprite sheet.
The numbers you see there are a part of my script so it may look different for you.
If you want every battler to use the same number of rows and frames, just make sure MNK_POSES_ACTOR is equal to {}
or in other words
MNK_POSES_ACTOR = {}
Should be how that line reads.
Hopefully this is the issue.