Are you sure it’s related to the iPhone X and not the version of iOS? Do you have thyou same version of iOS on your iPhone 6S?
Reason I’m asking is because iOS 11 brought lots of audio problems for MFi hearing aids like yours. I’m having tons of problems with my Oticon OPN 1. If you search elsewhere on the forums you’ll see others have same issues no matter what make and model hearing aids.
For example: Anyone having issues with Apple MFi Hearing Aids? - #66 by MrAerodynamic