They have all looked in good shape previously but will be looking at them all, starting from easiest to most fiddly, and cleaning them up in case

 

I would check the ELD.
At idle, the regulator is getting the right signal and the alternator is producing the expected voltage.
Then under higher load you're seeing the alternator output drop... but make the load even higher with the headlights and things are okay. Points towards ELD or wiring to the ELD.

 

Ah interesting, that backs up a diagnosis procedure I found that includes the headlights that made me ponder the ELD. Thanks!