I have never had that problem. Wonder why. I have a hundred or so salt water AL dives with old school bungees and no issues. Could it be the type valves that allow it to happen? I never weight tanks. Maybe it is where I have the rear leash placed?
Are you re-clipping (forward) the tank tails to hold them down?
I drop the bungie over the valve knob which is (obviously) pointing laterally out from my armpit. The rear clip on the tanks was clocked about 170deg from the valve knob. As the tanks get lighter they wanted to roll up and this axial rotation oriented the valve knob up at a ~45deg angle. This allowed the bungie to intermittently slip off the tank knobs. There was really no way I could figure out to clock the tail clips so the knobs would face out when the tank was both full and empty.
I eventually decided I didn't like moving the tail clips at all (I'm lazy). And I needed more lead than I thought in a drysuit; so I added lead to the al80s to keep the tails down. The only thing which bugged me is that the lead has to be on the bottom side of the tank tail strap. Any other clocking of the lead allows the tank to roll up, go out of trim, and continue to mess with the neck bungie.
I don't dive Al80s that often so maybe in time I'd have figured out a tail clip clocking which avoid the roll up. I love the old skool neck bungies with steel tanks, they are just less nice with al80s. If you have any pictures of your tail clips and their orientation that would be swell.