You have it backwards... The lighter cylinder weight would make the diver more buoyant, requiring less BC air and improving bubble stability.
This could be the point where the OP is getting underweighted and strugling on the edge.
It could also be that the diver is correctly weighted but trim is breaking down. The light cylinder could be reefing up on the upper body with a sensation of light overall buoyancy leading to the inflater dancce. Try moving 2-4 pounds higher on the body in trim pockets or on cam bands.
Pete
I bet it's this, compounded by the fact the diver was in shallow waters, where a change in depth of only 5' has the greatest impact on the air in your BC/wing.