Issue update follows. I'm gonna admit that I haven't read every response, but I appreciate all contributors.
I think I've gotten it figured out, but we shall see. (Update to ipdate: I don't have it figured out, and cell 4 is still going overvoltage.

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The other day I finally had time to get into this, and here's what I found. Opened up my battery crate, and started testing voltages. The cell that goes out of whack is the one on the positive end of the series. My crate does about 5 thousand miles a year in my van between jobs sites, and is never in a climate-controlled environment. I discovered, with my torque wrench, that all terminal nuts were just a tiny bit loose.
I also discovered anomalous resistance in the buss bar connecting cell 2 to cell 3(not the 3-4 connector, which would be more expected, with cell 4 going overvolt) There was basically zero or .oo1 resistance in all other bars, but this bar had around 6 ohms resistance. Pulled it off, cleaned with soap, recleaned with 90% iso alcohol. Cleaned the ring terminal for the bms sensor, even though I had confirmed that the sensor was detecting properly. Reassembled. Again, I retorqued all connections, and all required about 1/8th turn to reach spec torque.
I also went into the bms(jbd) and reset the capacity setting, which had repeatedly lowered itself to 210,000 mah.
Side note: I have noticed that one of my BMSs had increased its capacity to as much as 284,000mah over time, so the bms obviously has the designed ability to alter this number.
I left the errant pack offline for a few days, allowing the balancer, which is set to operate at all times, not just during charging, to do it's thing.
After reintigrating the problem pack, it balanced itself out with the rest of the bank. I've been running it for a few days and even though weather has prevented me reaching 100% soc, the problem pack is now apparently working properly. All cells are remaining in balance, and no fets have triggered any cutoffs. I did notice that the bms did lower it's capacity setting again, but not by much. Sadly, I forgot to note where it reset to, but it was a modest discrepancy. Around 272,000mah, or something. I've reset it to 280,000mah again just now. I expect to reach 100% soc today, so we wait.
So, I had several issues/questions. Here are some preliminary conclusions.
Can the JBD BMS reset its own parameters? Apparently, yes. Apparently this is by design and not a flaw. One of my 4 packs shows 204,000mah capacity.
Can an apparent "runner" be caused by other issues. Again, apparently, yes. A runner is not always a runner.
Is a connection issue usually visible? Not at all. There was no visual indication of resistance-causing corrosion on the nickel plated copper bar that connects my cells together, and even the ohm testing was open to interpretation. Testing from terminal post to terminal post produced variable, inconsistent results, but definitely higher resistance than any of my other buss connections.
I will continue to monitor. Hopefully, when we reach 100%, the bms will register good capacity. One of my other three packs registers above its rating/setting capacity. Pack three shows 284,000mah.
Thanks for all the helpful advice, group. And thanks for being civil. This group is far more friendly than social media groups. I don't know why. I love recommending this group to frustrated social media users. I hope they end up coming onboard.
May the sun always shine on your panels
