It confounds me as to how only the phono-stage can be affected whilst the AUX can sound as beautiful as ever at these times?
I don't see that as surprising. Unpredictable, yes. But surprising, no.
As you certainly realize, the phono stage uses circuitry that is not involved in the processing of Aux or other line-level signals. And that circuitry is vastly more sensitive, in terms of the signal levels it is designed to process, and provides far higher gain, than the line-level circuitry in the preamp.
Best regards,
-- Al