hi gary,it may not be the overboarding thats at fault here but only time will tell
as you said your bathroom upstair is fine with 9mm but there are issues downstairs,this is actually a good example to people why not to use an inferior product
your upstairs sub floor may be absolutely solid and tyhe 9mm has simply provided a stable substrate to
tile to but your downstairs has failed,if its the case the prep of floor is to blame say for example the sub floor downstairs wasnt stable enough then the 9mm wouldnt be strong enough to cope i hope you can see why it should nt be used even if 1 job has lasted