This is because your file tolerance is very low (0.01 Feet) for the size of the object you have and that the side surface in question got joined to the rest just within that tolerance… But when you split the object, the gap between the surfaces - gap that you don’t see because they have been forced together by joining with a low tolerance value - makes everything go sideways.
If you Explode the polysurface object and run RebuildEdges on it you will see the following:
I raised the file tolerance to .001 (for feet, .0001 may be even better), discarded the original surface, made a new one and joined all back up. Then Split works as expected.
Split Problem-fixed.3dm (173.7 KB)