Yes.
The Master Switch on the Power Dist Board isolates the batteries from the rest of the loom. For instance, the docs and EMEI state that any replacement of the batteries firstly requires that you turn the master switch off.
Stands to reason that driving with the the Master off (what you call the isolator switch on the PDB) would cut the connection between the 28v output from the front from getting thru to the batteries.
Also the test procedure for the PDB says the battery reading with Master Switch ON should say 24v with engine off then after starting the engine up the battery reading should rise to 28v when idling at 1500rpm using the hand throttle.
See EMEI attachments.
