The flux sounds like a good possibility.
Hindsight says, rinse thoroughly to remove previous chemicals before dipping in jewelry cleaner.

What kind of jewelry cleaner was it?
Or how old was it?
I've had some old silver cleaner make my silver sort of yellow when the silver was dipped for too short of a time.
It was kind of like it "redistributed" the tarnish instead of actually cleaning it.
When I soaked the charm bit longer, and swished it around a bit, it finally came clean.
I also had to rinse it very thoroughly, for it to remain untarnished.
--Polly