It’s possible to have a pressure loss between the pressure tester and radiator. I’ve seen this more than a few times. You might consider wiping the connection surfaces down with a cold, heavy grease as this can help stop any pressure loss. Stick the grease in the fridge for a few hours if need be. (Yes, this can cause disgruntlement with a better half.) 
What about the pressure cap? These can be a hit and miss thing sometimes and maybe coolant is being lost on an erratic basis while the car is at speed on the open road.
We had a Subaru in once with a coolant loss complaint in which everything checked out fine; multiple times.
Come to find out there was a narrow window (between 7 and 8 minutes) in which the pressure cap would allow the radiator ot puke anti-freeze for about 20 seconds or so.
It would not do it at the 5 or 6 minute mark or when in excess of 8 minutes. A new cap solved that.
Glad to see you’re still around GS. It’s been a few years.