From my experience, U12t needs to be snug in your concha so the tube is deep in your ear canal. I needed a lot of tip rolling to find best fit. Some were disastrous!I also found out that changing the ear tips change the tonal balance massively.
I think it has something to do with TIA driver that's in the tube itself. Driver needs to be as close to ear drums as possible or it will negatively affect treble (maybe that's the issue for you?).
TIA's purpose is to give that special airy feel in the upper-treble frequencies. I speculate bad fit can shift those frequencies to lower frequencies and/or cause unwanted resonances.