Air? in upper floor Rads bled but comes back (bit like me with sprouts) Sealed Combi brand new. Useless Plumbob!

[DIWHY]

Yes I know this question is donkeys years old on here.
BUT no clear answers, Chemists or Plumbers welcome, not the clown who fitted mine as he's gone to Thailand, best place for him.
1) My brand new sealed pressurised Combi system has no leaks but seems to replace any air I bleed away just on the upstairs rads.
2) The Rad (mainly just one, sorted the others ok) is cold always on the lockshield valve never the Thermo valve side.
Pressure is stable and fine.
Most rads are new, I did them, I'm a retired qualified nuclear pump engineer.
Any words of advice thanks

 

[oldbuffer]

Test that it is air and not hydrogen. The latter can be released by chemical action within the radiators. Usually indicates a system which needs cleaning.