I once sawn a 3/4 pipe laid along a footpath aprox 1 mile to a stop cock on a distant property & not a drop came out the end, yet there was 100psi at the meter. This bloke is talking about 12psi though 800ft.
Also when we used low pressure “Tank Pressure” we sometimes used to get air locks these were in in straight copper pipe. PEX pipe bends like crazy! maybe a jetter to purge would work?