Those things are surprisingly sensitive to air.
I might be mistaken but I think they possibly vacuum the bottles before putting the seal on them.
I know decades ago there was a product in a can that you'd spray inside the bottles to remove the air and make the chems last longer. I wonder is fanned air wouldn't do the same since the can contains no air to begin with?
I think I remember that Francois, it must have been CO2 in a can because it is heavier than air and would displace the air. For my next batch I'm going to get phenolic caps with the inverted cone shaped LDPE liners, they are advertised as gas tight. I think I'll mix into distilled water, who knows what's in city water.
Seen those collapsible plastic bottles too, years ago. Thought at the time they would be hard to clean. I generally like and use the amber Boston round bottles, in 16 and 32 ounce sizes. Another thought is to order a dozen gas tight 100ml plastic bottles and decant the concentrate into them, then just mix one quart at a time.
All summer long, I can't print, the bathroom/darkroom gets too hot and stuffy, so I wait for the months between November and March although I will nick in occasionally to develop pinhole paper negatives because that requires minimal equipment and set up time.