I have a similar issue. My 6 and my ipad mini sync the recent call logs even if I leave the ipad at home and make a call when off my wifi and out of my house. BUT my wife has an identical 6 and mini running the same 8.4 release as me and hers will not do this. We have tried to get it to work on hers, and it simply won't. Any ideas how to make this work? again, not wanting calls on one phone to show on the other, but calls to/from a phone to show on its associated ipad.
For the record I was still using an Ipad 2 at the end of May when this first popped up on my facetime logs. when I restored my backup to the new mini it has kept updating call logs almost instantly.
and no we do NOT share apple IDs between us, each have our own. I use my id on my phone and my ipad, she uses her id on her phone and ipad. Family share is on since both kids also have 2 apple devices each(and their own ids).