When you tested, was the 24V battery charger running via inverter?

Why do I stress this?

The regulator will regulate what is coming out of your alternator, NOT out of your extra charger that is essentially in parallel with the output from the engine charging system.

IF it does not like the modified sine-wave from the inverters, it's possible that the charger could put out a funky 24V, with a large AC ripple. I don't know, but this seems to jive with Tom's experience.

IF you have a large ripple right on the battery posts because of the charger, your dash is going to see this.

28V out of the regulator is fine.

So, without trying muddy the waters, did you turn off the battery charger? IF so, did it behave as expected then? If it's still misbehaving, then you've ruled it out, at least.

Ray