The point of no pressure change is where the EV connects into the system and is the EV pressure itself, with the circ pump off then this pressure will be exactly the same throughout the circuit, after allowing for elevation. With the circ pump running, while the pressure where the EV connects to the system will still be exactly the same then its possible to get negative pressure(s) somewhere but only due to pipe friction losses due to blockages?.
Maybe all combis have their EVs connected in likewise, someone might care to watch their own boiler pressure gauges on start up and note whether it rises or falls with pump start and also its magnitude, the one above certainly moves more than I would have expected, even though theoretically possibly correct, possibly pointing to restriction somewhere?
If the system isn't ingesting air then all this air must be trapped since initial filling but as its being slowly vented then the boiler pressure should gradually fall and require top up but it isn't. Perhaps drain the whole system again and fill very slowly stopping to vent from the bottom up.
The only other thing maybe worth checking out is the PHE, if it had a pinhole maybe possible to keep the system nicely topped up from the mains and if its at or near the system pressure then no problem with boiler PRV lifting. but plenty of oxygenated air gaining access.
Suggest shutting off the mains feed at the boiler and open a hot tap and leave open for a few hours with CH in service and see if any change in behaviour, presume the diverter valve isolates the primary water from the PHE but something may still show up.
A interesting riddle alright.
