Cylinder drift with new seals

Somewhere on here is an extensively discussed post about cylinder drift with valve closed which I cannot find. I found a similar thread from 2012 which discussed some of it. Here is my problem. I just resealed ALL cylinders on a Case 580D TLB. The problem cylinder is a stabilizer and was the second unit done, using a ring compressor and oiling the piston before installation. It is 90 miles on a service call from home and my wife and I finished right at dark and performed a quick operational and external leak test and all was good. No leaks, all performed correctly or so they appeared. The next day, the owner texted me that there were no leaks, but the stabilizer was "leaking internally". In 5 minutes or so, it will leak up from applied/extended, and down from retracted/stowed. with no external leakage. Down from retracted with a leaking piston seal is possible as it would create a vacuum bubble on the barrel side, but up from applied would require by hydrostatic principle, an external leak, which the owner assures me it isn't. I know this to be the case as I recently had a farm loader that would raise and HOLD with some sponginess after valve closing with cold oil, but after the oil warmed and thinned, it wouldn't raise at all. Upon lift cylinder disassembly as they were the only possible culprits, I found the piston seals to be completely gone and only the close tolerance of the pistons to the barrels sealed it enough to raise while cold.
I had the offending cylinder barrel tied up for assembly and it went together perfectly so I don't think the piston seal was cut. I have ordered another seal kit and will go back on the 31st and investigate, but this doesn't make sense. A leaking valve is unlikely, but I don't see what else could allow this to happen.:confused:

Cylinder drift with new seals