One of the challenges in determining the cause of the problem is that the pressure maintenance pump and its submersible motor are both located beneath the pump station skid in the concrete vault known as a wet well.
From ground level, this is what the pump station skid looks like:
Peering in the access hatch on the right side of the skid, you can see that the two main pump discharge pipes are on the sides, with the smaller pressure maintenance discharge pipe and its electrical supply conduit located between them.
It looked like we were going to have to have a service truck come in and pull the steel discharge pipe, pressure maintenance pump and motor up out of the wet well--not a quick, easy, or inexpensive solution.
However, before going that route it seemed like a reasonably nice day for a dip. So, into the wet well I went, and for my efforts, the treasure I was rewarded with was...a plastic shopping bag wrapped around the pump's impeller. Not sure how it got there, but that's the kind of quick fix, we'll happily take.
Now this flow looks much better: