Low ambient control needed

Do TXVs have to have a minimum pressure at the inlet in order to work as designed?
I understand by lowering head pressure then efficiency is increased and that capacity would be decreased but how low is too low a head pressure for the valve to work as designed and not cause problems.

In the winter times I see so many units especially the air handler types with internal and external units that are running at a head pressure of 10-11 bar( fixed speed compressors)
just basic units but because the indoor fan is a fixed speed fan it just runs at a constant speed and can't regulate the HP

The evap coils are always freezing up even *though the return air from the building onto the coil is usually 21-22degrees.

Gas charges are ok subcooling is around 6-8k and superheat is always being controlled at 6k and no moisture present in the system, other than the coils freezing up the systems heat and cool fine.

Other than the humidity factor all I can think is the low head pressure could be affecting the txvs but not too sure.
Can any one shed any light on this for me please.