“”””One thing I learned the hard way, having worked in ‘the business’ a while ago, is to be absolutely sure that the flux you use is fresh, as in recently purchased at a high-volume outlet, as using old flux is not any different than using no flux at all.””””
I’ve used old flux kept in my plumbing van for years in a hot climate and never had a problem.
Being a repair plumber, my can of flux with the hole in the lid and a brush sticking out would be used for years and the old backup cans that had been stored in the van for years then being opened.
Could be the brand I attempted to use at the time. I finally tried another can, but much newer, then no problem.