Thanks for the help – it turns out the dialogue levels weren’t the culprit.
It was a strange issue of panning… FCP defaults audio pans to -1.
After trying a million other things, I randomly center panned all the dialogue and ran a quick export and upload test. Watched it on mobile and – CLEAR dialogue.
Center panning it lowered the overall volume quite a bit, so I had to boost everything to get it back up to par. It must have to do with the way it was recorded, which was out of my control.
I didn’t know panning could have such a muffling effect like that – and on mobile only.
Very strange issue, but…
SOLVED!