I don't recall where Apple ever used the words "backward compatible." Apple said it is Qi Wireless charging capable, which it is. Can you point out where Apple actually used the words backward compatible?
Not ALL Qi chargers are created equal. There are single coil. Dual coil. Stand. Plate, etc. Some don't have QC3.0 bricks. Some which do, have lower watt bricks. You need one with at least 18W and in the case of MagSafe, it won't work unless it has a 20W brick.
Why you're still having restart to get your phone to charge suggests to me, it's either an alignment issue with not getting the coil aligned well with the new phone, or it could be a lower watt brick, which just doesn't output enough power to reliably charge your phone consistently.
What I do know for sure is that replacing my older chargers as I wrote above solved my wireless charging issues.