Just a question... where is your temp probe mounted at ?
It appears like it is on the suction line just behind the evap, I say this because -55.9C on a single stage should not be possible, I might be incorrect. In all the years I have been running SS Phases units I have not seen a unit run at those temps and even be remotely able to hold a load. Most use R507 now, or even something different as in the states I believe they changed what could be used. Will have to get a pro builder in here to give feed back on the temps. I believe temps actually get colder as the gas gets pulled through the evap into the suction line, but that's not the temp inside the evap near the CPU.

The probe should be mounted at the base of the evap, near the CPU, some even drill a hole into the base of the evap and mount it there.

I have a monster SS Stage built by Ron, it pulls -45C with probe mounted like I said.