Was reading some posts, very old posts (2012) on the UK forum regarding the F5.
Found this interesting, don't know at this point if I ever included in the F5 Tips and Tricks.
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A 2" nail with a point, on a Fisher machine (and often other machines if set to run as hot as the field will allow) can and does create a false signal (Iron grunt/nice hightone)
So if you have the iron notched out you will only hear the good hightone part of the signal.....
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Fisher machines unfortunately are often fooled by nails/large iron...
I see you have used Minelabs, they too are fooled sometimes, but that depends how you run them in the field...
I have an Etrac and the FisherCZ3D.....

The problem the OP seems to have is notching iron out....notching uses a huge amount of discrim to completely take the iron signals out.....
Trouble is by telling the machine you dont want to hear any iron signals, does not tell the machine you also dont want to hear the falsing part of an iron signal......
Without iron notch on you would hear the iron grunt and the hightone of the false (if it did false on that particular piece) then you would know its iron and move on....