I have made crosses involving Northern Bluegill with both Redear and Pumpkinseed; a lot of them over the years. Color pattern on flanks show spotting and opercula show radiating lines, both characters of Pumpkinseed which the Redear lacks. If Pumpkinseed are rare, then that sets the stage for hybrids involving it. Somebody does not have a good handle on what species are present in the lake.
Is this the type of "spotting" you are referring too? We are at the tail end of the primary spawnining season here. I'm confused, do you have pictures? Pumpkinseeds are rare in SE Nebraska. GSF on the other hand are in most watersheds, it would not be uncommon to find a few in new or renovated lakes around here but these do not look like GSF hybrids due to the elongated pectoral fins.
