I have the same issue with a Wireless AC-7260 on a D54250WYKH and this problem is there since these cards appeared on the market.
Anybody who owns one of these Wireless+BT adapters have the exact same issue.
It's not related to anything else, the problem is with the bluetooth adapter and it needs a working driver, but as I said this issue is there for long months and there are other threads about this (like mine over here), but after some common answers (e.g. telling to update the bios, drivers etc.) usually nobody gets a usable answer about this serious bluetooth problem.
As they said, Intel engineering is working on this matter, but nothing happens and the problem is just there as it was before.
PS: This bluetooth problem appeared three times while I wrote this answer.
Okay, so just to be really sure that the problem is with the driver, I completely turned off my router, phone and everything else that uses power and closed the windows and the door.
I turned on the NUC in front of my bluetooth mouse and started to move the mouse pointer around, but after a few minutes the pointer stops, so the problem is still there.
By the way, I don't have any log entries when this happens, but you can check the other things I tried here, but nothing solves the problem and as Allan said it's a known problem and Intel engineering is working on this matter, but how long? Because the new bluetooth driver still have the issue.
Is there any beta version that we could try?
Could you please contact the engineering team and ask them about the progress?
As I said in my original post "sporadic connections occur even in close proximity to the NUC". I can literally have the keyboard/mouse touching the NUC and the problem still occurs. Also, the AC-7260 in my Thinkpad T440p works fine with the keyboard at any reasonable range.
As I have ruled out all other possibilities, this is either a defective wireless card or a driver issue.
What I really need to know is should I RMA the unit or does Intel have a fix for the bluetooth problem?
I totally understand your frustration and believe me I would feel the same if I were affected with this problem.
Checking the thread I see you have done many important troubleshooting steps so I know you have heard this before but we are working towards a solution, we are investigating and trying to provide the solution for you as soon as possible. At this point, we do not have an ETA but I will keep you posted.
In the meantime, I am going to research more on this so that I can provide more troubleshooting steps.