I agree flux can cause issues when used incorrectly, I’ve had a number of returned items come back for repair absolutely soaked in residue and it’s quite nasty to work with.
Although similarly to Bob said, generally the outcome is not so dramatic on a soldering iron, my 2c is that this is either an arc as Bob suggested, or potentially from excessive temperature. Depending on whether the model includes thermal runaway protection, if the thermistor in the iron fails, it will continue to heat up until failure.
In this case, if that’s occurred it may cause the metal to become brittle and easily shatter as you’re essentially creating small amounts of martensite when it cools relatively quickly in the air from high temperatures each time it’s being used or if you’ve hit a cool liquid with it. (Also, that discolouration is usually due to post-processing on the steel and is usually more prevalent with less expensive irons, it only occurs the first few times)
As James suggested, the Hakko Irons are good value, we actually use that here for prototyping with our PiicoDev and Makerverse range as well as for general fixes around the warehouse and office.
Personally I really like the TS-100 as since it’s so light and small it provides for quite delicate control over where you want the heat and components which is essential for any work on SMD scales. The fact you can power it via USB-C is also very useful when needing to work remotely.