Same deal with my 27DC trolling type (deep cycle-starting) batteries I had. Drove me nuts.
They would hardly bubble at all when gassing (unlike antimony 6s) so I don't think they were getting destratified under normal charging. They still had their capacity and voltage, so IMO that was with the "good stuff" lower down out of reach of the hydrometer. Stronger acid localized like that eats plates so it isn't the best arrangement.
I used to tip them on their sides to get a good mix. I kept them in the garage at home between trips doing recoveries on them-- PITA. Not convenient for your situation
I don't have any bright ideas how to solve that, sorry. I went the 6s route to get away from all that.