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I personally pull a little of a angle, not straight up, to try to help avoid this result.
but if/when it happens, I simply use a clean/sterilized tweezers and push down the metal tightly against the stopper (see pic below).
I often have to do this regardless because even if the metal is not bent, there are often times little sharp edges from where the cap was connected -- I push those all down tight and flush into the stopped so when I wipe with alcohol pads the material does not catch and snag on the metal tabs.
Simply use a clean metal implement (such as tweezers) and sterilize it with alcohol; then, push down around the metal, every millimeter or so, rotating around the circle until it is all pushed flat against the stopper.
then it will give you a nice smooth and flat surface work from when you whip and sterilize before each use.
Nice delivery time by the way, thanks for sharing your experience.
I do not have much experience with the mixes; I hope you enjoy mix-4.