If previously, with the same charger and cable, fast charging worked properly, this is the same problem I had a few months ago. In this forum thread you can find some possible solutions… for me, it worked cleaning the contacts with isopropyl alcohol and a cotton swab.
But, if the phone is still under warranty (which I assume is the case), you can #contactsupport and maybe they can help you.
I hope it helps, best of luck.
PD: This problem is also being discussed here.