I've worked with isomalt several times and have never had it turn amber (like overcooked sugar),especially at 260 degrees. It is about 2 yrs old (not opened and in another ziplock) and I live in a very dry area.
Maybe it's my candy thermometer(?), though it's not that old (3yrs maybe) and good quality. I don't use it that often - 2-3 times a yr.
Any idea why this happened?