Weird no-stereo audio issue on headphones only

This is a weird one! On headphones, all audio sounds like it's out of phase, very garbled and muffled as well.

Whilst debugging I found that if I moved the L - R slider in Accessibility just a fraction off the center, audio then sounds fine but with what appears to be no stereo separation. I ran an audio test track and sounds that should be in L or R channels are all coming through both speakers.

Playing through the loudspeaker it all sounds fine but of course this is two channels being fed through a single speaker so may mask the issue?

I'm lost as to what might be the cause here. Is it worth replacing the dock connector assembly perhaps?