Hmmmm, interesting. Agreed sounds like its starving like something didn't switch completely.
Is there a special procedure for the switch between fuels? Sounds almost like the carb is being gummed up with old fuel sitting in it while it runs on propane, but your pictures show otherwise.
I always run either seafoam or stabil in all my gas engines and my gennys always get the fuel valve turned off and I let it run till it stalls.
Preaching to a guy who makes a living with gas combustion engines and owns a racing team, just thinking out loud lol.