Mine is also very slight—this is likely because the “detune” is coming from the ambient, not-precisely 0V voltage present at the jack with nothing plugged in. My impression is that this is not something expressly controlled for, so some units will see more effect of this than others. Of course, you can always explicitly recover this functionality by patching some constant voltage into the fm input.
All of that said, one way to double-check the effect on your unit is to try sound/cycle mode, listen to identity and one other output, set intone someplace where you can hear two distinct tones, and then move fm from noon to fully CCW. the pitch from identity should remain constant, but the pitch from the other output will change very slightly.