Many years ago, I had one fuse onto a valve as well. It happened over winter and salt is the reason.
Later, I found white lithium grease inside the cap works to prevent this (available walmart, a tiny can lasts years), I’ve used fancy caps of all kinds for a dozen years without a repeat. Note I switch between winter and summer tires and I remove the caps before storing the tires. I won’t leave the caps in year round even with the grease.