As found e.g. here, some configurations of GTK seem to have issues with deadkey-based keyboards.
As a fervent user of the “US-international (deadkey)” keyboard, this was very annoying. Specifically, it was impossible for me to type a quotation mark (“) because redshift invariably changed it into an umlaut (which does not even exist stand-alone on this keyboard!). Sadly, there is no easy way to solve this.
My work-around consists of an autohotkey script, which sends the correct unicode whenever you type “, rather than letting redshift handle this keystroke. It works fine, but it does stop the umlaut from being used in deadkey combinations. As such, it’s mainly intended as a primary stopgap until I figure out how to make AHK handle this gracefully.