I had the same problem, seems quite unbelievable that Samsung don't know about this and fix it, what are the chances of just stumbling across the fix.
Samsung, so good in many areas but totally useless in others.
Thanks for the fix info.
Re: Unable to place calls over Bluetooth when Android phone UI screen is locked
This section had a default setting of "ONLY WHEN DEVICE UNLOCKED". Changing it to "EVEN WHEN DEVICE LOCKED" completely solved and eliminated the issue I was experiencing. Oddly and gratefully, the new setting did...