I think your comment about machining tolerances/factory errors might be coming into play here...possibly, certainly not out of the question for a hardware error.
I must ask: Have you checked how tight the positive post is ? (+ insulator that sits below it) on the deck ?
It just occurred to me that they might not be all equal when it comes to QC on the assembly line
(I know it sounds a weird concept coming from a Chinese factory - but I`m quite serious

)
Wondering if your positive post on one wasn`t quite as tightly screwed in place at the time of assembly.
If it`s in any way loose it`s quite feasible that juice could be getting inside there quicker on one unit than the other.
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I`ll give these 4 test tanks until Friday evening
(3 days, my usual time allotted for a coil to live, that`s a guideline not a law

- any longer is a bonus if it performs OK still)
if all is going well, within reason, I will change out the "extra" o-rings for some slightly larger ones if I have any and do another test run, maybe with just two this time and see what happens.