912 ULS Fuel System / Momentary fuel pressure drop

I think you'll be fine.

The Fuel return line should be routed to the Fuel Tank instead of the Gascolator.
Turning ON the Aux Pump may be pulling the Vapor Bubble out of the Gascolator.
It will take a few seconds for the Pumps to clear the bubble.

If you suspect an Electric Pump problem...
With the Engine OFF, What fuel pressure do you see with ONLY the Electric pump?
This should be a pre-start Checklist Item,

The engine needs a minimum FLOW Rate capability that can Not be measured in Real-time.
So we measure Fuel Pressure instead, as an indicator of the Flow capability.
The engine will run just fine at 3 psi, especially as a momentary pressure drop.
It does not matter what the pressure is as long as the flow is sufficient.
It will even run for a short time at Zero pressure until the carbs go dry.

The 912 equipped Air Cam requires the fuel return line to go to the gascolator instead of the tank. I get between 4 and 4 psi with the aux pumps on and off.

In farther discussions, we suspect the builder placed a restrictor in the return line by the manifold and this is what creates the temp drop in pressure. We will investigate this and see if its true.

Placing " ...a restrictor in the return line by the manifold ... " is the correct location for the "Return Orifice".
This will cause the pressure to Increase slightly with higher flow. (Aux Pump ON)
A restriction (or Air) in the Supply-side can cause the pressure to Drop with increased flow.