That's an unfortunate reality for these pumps.
People say that vinegar is the reason for these to swell up. But I have an alternate theory.
These neodymium magnets are extremely brittle and they're encased in a relatively thin barrier of plastic.
When we clean these things we don't consider the strength of the magnet and we often times allow them to slam down onto a metal surface like a sink. All it takes is for the force to be strong enough to crack that magnet and in turn create a small fracture in the plastic that allows water to make contact with the magnet. After that, it's game over.
I have conditioned myself to be extremely mindful of this when cleaning my wetsides with vinegar, after 2 years of this, I have yet to have a swollen magnet. Knock on wood.