One explanation for the "sucked-in" airfilled and "bulging" butaned bottles could be temperature changes. I do have a very old unopened bottle of Ilfosol-S whith sucked-in sides, so oxygen absorption is definitely taking place...

When you fill up a bottle wit hbutane, the gass expands. That cools it down. When you close the bottle, the butane is still below ambient temperature. As the gas warms up, it expands - bulging bottles!