Does the voltage settle at 24V on the button or does it fluctuate at all? The open circuit voltage of a 12V panel is about 21V. But in order for you to read OC voltage on your controller, the panel would have to disconnect from the load, I can't imagine how that would happen when you plug into shore power. Unless, there is a relay that operates when you go to shore power so that you do disconnect the panel.
Has it always operated this way or did it just start?
I've got 6 135W panels on my rig and have never seen that happen when I plug into shore power.
Paul - WA1IWH
Margaret - She who must be obeyed.