You're right, I'm not sure what it was, but one of my strobes was always firing in TTL, regardless of anything, while the other was sometimes doing a single flash instead of double, resulting in a lopsided exposure, and the right magnetic switch sometimes had an effect on it, and sometimes not - and it seemed different between on land and at depth, and sometimes turning it off and back on fixed it.
For what it's worth, I was able to figure out this issue, eventually. The fiber optic sockets in my housing are offset to the left from the flash head, and there is a little reflector guiding the light to the left socket, which ends up not aligned with the flash (the right one is fine). The resulting weakened pulse is sufficient to trigger the attached strobe on the stronger main flash, but not on the weaker pre-flash. Fortunately, the fiber optic cables that shipped with the strobes had a double FO plug in each box - I disassembled the Sea & Sea type L-shaped plugs on the cables, put the cables into that plug, and now both cables go into the properly aligned socket.