I was seeing the same thing until I figured it out. Not sure how to explain it. Running Pro Zero with no iron disc (set at 00) you will hear the low tones on low conductivity targets such as iron. However, if you turn on Iron Audio with the iron disc set to 00, your mid tone will now extend all the way to zero. This is because when Iron Audio is engaged, the low tone is only given for VDI readings below the Iron Disc setting. If you have it set on 00, then there is no place for a low tone to sound and it will seem like Iron Audio is working backwards.
I just went outside and re-checked this. Swing over a target with VDI 27 and get a low tone. Turn on Iron Audio and that same target now produces a mid tone. If I raise Iron Disc to higher than 27, that target will then get the low grunt Iron Audio tone.
I hope this kind of makes sense.
Papa