I have all modules except combined arms and the MI-8. This issue is present for all modules I have tested but more noticeable in the jets and P-51 (moving faster so easier to notice).
I have done a full, clean reinstall of 1.2.6 and the issue still persists.
I also have 1.2.5 installed and the issue is NOT present.
My TrackIR is also up to date.
I have experimented with vsync on and off (both in game and nvidia active vsync) with no noticeable improvement.
The issue is not related to the movement of the head being different in 1.2.6, but the stuttering and screen tearing effect that it causes.
This is definitely, 100% a bug.