So here's an interesting update... I've been shooting the F6 with a Sigma Art 50 1.4 - and about 50% of the pics were out of focus. But that lens is in focus on my D850, which with its high pixel sensor is far more sensitive to focus issues.
I then replaced that lens with a 50 1.8G on the F6. And again the pics were OOF. I then noticed that in the VF, when the focus confirmation dot lit - whether in MF or using AF, the view in the VF was obviously OOF. So it seems that the focus in my F6 has drifted/been knocked off. So much for this only happening to RF cameras...
I have been extremely careful with my F6, it has never been bumped, does not have a mark on it, looks new. So this is very disappointing indeed, especially since I picked it as my 35mm film camera on a photo trip at the end of last year when I first noticed possible issues (once I got back and saw the results).
At least this camera is a gen-you-whine USA version, so I should not have an issue getting it serviced. Nikon USA does not touch grey market cameras. But this is $till going to hurt..