Did overfill protection worked? (5.9 gallons)

[RT1997]

My new Manchester propane tank has the same 5 gallon capacity as my old one on RT 170 .And it is supposed to be only 80% filled. So if overfill protection works the shut off should be at 4 gallons or slightly over.
I am not sure how well that protection worked on the old tank but definitely at 5 gallon mark at the filling station
the RT tank "refused" to take any more and the rest were coming out until the tech turned off their machine.
I usually monitored the process and asked to stop according to the gauge readings.
I recenly went to a propane place with new Ferrell Gas high tech filling machine that supposedly stopps itself when necessary.I don't know the details,that was what a tech told me a while ago.
Anyway,I was counting on my tank overfill protection.
So the process went seemingly fine.The machine stopped.No hissing propane in the process of detaching the nozzle.But it stopped at 5.9 (!) gallons.And the gauge on my tank pointed far over "full" mark.
I know that Manchester tank can hold more than 5 gallons.
But there is the gauge and 80% requirements.
Of course from now on I 'll myself become the gauge )): and monitor the process.Even if the overfill protection is faulty shipping the tank back to Canada ($$) and paying another $$$ for installation ( even if it might be less than 5 hours second time) is insane .But would be interesting to know your opinion on how did that happen - did Farrell Gas high tech machine "overide" RT tank signal ?
Again- weather or not the machine is really high tech I only heard from the propane station employee.

Side question - any possible damage from one time overfill?