Did you also experience the distorted sound with the Zircon 1 model and custom molds?
If not, then i assume the distorted sounds occuring due to oticon feedback management , while you were wearing power domes with OPN1 and Real 1.
Because custom moldes having a better tighter fit and the receivers are sealed inside the mold.
Whereas power domes are open to the outer ear and sound/resonance can leak outside from the receiver/speaker easier than from custom molds.