I would gather more data than just high head/low superheat before concluding non-condensables are in the system.
High head/low superheat can be present in a system that is clean internally (no moisture or non-condensables) but has dirty outdoor coils and restricted airflow inside. If anything, non condensables when they reach the metering device would enter the evap and rob space from the refrigerant to do its work.
Evap pressure would likely run higher than normal due to non condensables (law of partial pressures), aggravated by high head pressure and a high pressure stacked against the metering device. Higher evap pressures is a condition not often associated with low superheat (due to airflow problems), unless the system is ridiculously overcharged and the mass flow rate is mostly responsible for the low superheat.
Electricity makes refrigeration happen.
Refrigeration makes the HVAC psychrometric process happen.
HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.
I am not sure if there is definitive answer since systems are different. It could be either thermal/pressure or thermistor controlled expansion devices or fixed orfice. Each one would be affected differently. I had a 10 year old miller/nordyne 2 1/2 ton split system (fixed orfice) the other day, I could not get any good readings superheat and subcool readings were way off not even close, the superheat reading was within 2 degrees of saturation with no change after recovering most of the charge. I finally just recovered ALL the refrigerant/evacuated and recharged with virgin R22. The only conclusion that I could come to was that someone had topped the system off with some weird refrigerant (possible R134A as it seemed to have a higher temp to pressure ratio) obviously it was not R22/or it had air in the system big time maybe?. Everything was good after recharge perfectly normal readings. Could have been air? I do not have a refrigerant identifier I wish I did it is on my wish list. If you are having weird readings I would suggest recharge with known good refrigerant kinda like these silly computers if they get corrupted sometimes you have to drain the juice and restart them LOL
When you give a lesson in meanness to a critter or a person, don't be surprised if they learn their lesson.