It happens because unlike radiators there is nowhere for the air to hide (top of rads) and because it is hotter (more air dissolves out of water the hotter you get it).
What can you try?
- Remove the air
The system needs to be bled properly. Heat the tank HOT, then heat the radiators HOT, then flush all the air into the radiators by alternating between heating and hot water (heat pump off, pump on, cycle the diveter), and bleed the air from the radiators.
- Design it more robust to air
Mounting the flowrate sensor on the supply side (not return) after the system pump and increasing the static fill pressure (2 bar, if your system can take that) will both reduce cavitation within the heat meter’s flowrate sensor.
- Sanity check there are no mistakes
You also need to check the position of the flowrate sensor isn’t somewhere that it will be adversely affected by air. The meter installation manual will show where it should and should not be fitted.