It seems to be a hardware limitation, sorry to say.
I did some googling, and it seems both wifi and Bluetooth can interfere with each other.
If you have control over the wifi network, you can try switching it to 5Ghz and see if the problem persists.
Did you test on stock ROM?
EDIT: Also, it looks like, if I disable wifi optimization things work fine. I had issues even getting my headset connected this morning, but after disabling optimization (with WIFI off) it connects right away. Also, just to note, I am on the 12-15 nightly, not your custom version... was going to give that a shot next