Really return volume at different loads and rpms comes down to the pump, especially the advance piston wear. You may see the most at idle or the most at low load but high rpm, or high load high rpms, just depends on the pumps condition and efficiency.
The injection pump, I'm guessing.
Considering I had zero problems before switching, it must be something directly related to the Holley conversion, not an IP problem.
Doesn't the filter head return line connection have a check valve? Perhaps it's weak and is bleeding pressure from the supply. Just a thought.
The fuel pressure drops when shut off, as if it's going somewhere. Wouldn't it normally hold pressure? Maybe it's how Holley pumps just are. I had no starting problems with the mech pump so it wasn't losing prime.