If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.
Pressure is now between 2.2 and 2.5 (I've not seen it drop below 2.2).
There's defo water in the rads at the top of the house.
When I pressurised to a higher bar I got the tado app to open all valves so everything was open. I only forgot about the two rads in the house in the hall that have normal TRVs.