OK, I have a Note 4 on Sprint. Fast Charge for me is also "wonky." Here are steps that so far have worked as a workaround. (Full disclosure, I almost invariably charge my device when off, and these instructions apply to phone off charging):
1) Turn off phone, wait for black screen.
2) Plug in fast charger, from the wall to the phone
3) Wait for the screen to go black again.
4) Click the power or home button to get the screen to come on (shows the one dot in the bottom center).
5) Unplug the phone and plug back in before the screen fades to black
This has worked on at least 4 occasions, but I can't guarantee this will work for all. Please let me know if this works for you!