RV12iS, Fuel Pressure low after engine start?

What is your Bus voltage before and after start?

The fuel pumps strain when the voltage drops much below 13v.

How long are the wires from the Engine harness to the Fuel Pump switches and what gauge wire was used?

16 awg is the minimum recommended for short runs. (5-6 ft)

You might want to measure the actual voltage at the two pin connectors on the pumps.

Bus voltage before engine start 13.1 V, with engine running 14V, so this is not an issue!?

I get way more pressure than that before starting - around 46psi with both pumps and about 13.1 volts.  44 and change with one pump. If I were you I’d check first that there isn’t a major blockage somewhere, if not, check the pressure sensor.

With the 912is engine the fuel pressure regulator operates with respect to manifold pressure (MAP).  So your fuel pressure gauge needs to either reference the air box pressure or be corrected for this on your display.  When your engine is off with the pumps running, manifold pressure equals outside air pressure so the gauge is accurate.  

When the engine is running at idle, the MAP is much lower than outside pressure so the gauge is going to read low. When you increase rpm the MAP increases and the gauge reads closer to what the regulator is asking for based on MAP.  At WOT your MAP is once again very close to outside air pressure so the gauge is very close to correct.  

There are fuel pressure senders that allow you to connect the reference source to the air box, which will then give the correct reading. Also, some electronic displays will have a setting that lets you use a standard sensor and correct for MAP.

Thank you very much, that makes sence!
I measured 156 liter / h fuel flow, so, the pumps and the fuel lines are ok.