Hi Phathom,
I am a long time Nikon user, but I am having an issue for the first time with the combinations of the D750 and the D800 with the Tamron SP 70-200mm f2.8, USD Di lens.
By the way, I do have the Tamron SP 24-70mm f2.8 USD Di lens which doesn't give me this focussing issue.
My issue on the D800 and D750, with the 70-200mm is:
When focussing on low contrast areas, where the lens would normally shift focus from close to infinity and back again, I hear a long "beep" and the focus locks. The manual focussing ring seems to be what causes the beep, as it actually starts to move as I hear the beep. This I checked after getting my assistant to press the shutter while I watched. The beep is definitely from the lens.
This sometimes happens after one or three presses of the shutter, so it can be quite disconcerting.
After the focus on the lens locks, I can press the shutter and a picture is taken, but it is out of focus.
The only way I can "unlock the focus", is by switching off the camera and turning it back on. This is unacceptable, especially if one is trying to capture a special moment at a wedding or sport event.
I will be trying to get the shop to replace this lens.
Cheers