Stereo/mono issue with a mix

I've just finished a hip-hop mix and checking it across different playback devices.

I noticed that on my phone, my mix sounds great, the vocal is strong, the levels are perfect, everything is glued nicely, it matches up to the balance of my reference track... if I listen in landscape.

Once I hold the phone vertically, the mix becomes a bit choked with a tiny "hole" in the vocal, there is some apparent distortion and a bit of unwanted grit.

I'm guessing it is an issue related to a mono fold but I'm pretty much clueless as to how to fix this without affecting a mix that sounds almost perfect in stereo.