Yes! It seems the problem is not related to the approach at all, but AFTER landing, during roll-out.
It seems that the aircraft enters a side-slip whilst on the ground and using rudder to maintain the center line. The wheels seem to have no grip on the ground, with the aircraft acting like it is still flying.
Maybe the landing is such that the sim thinks it is still flying and applies the physics of such?
I had it occur again last night after I applied a new procedure of not touching the rudder on approach more than necessary. The approach was fine to touchdown, then during braking the aircraft mysteriously slid sideways (like you applied a force from the side and pushed), requiring a boot full of rudder to swing the nose around (which made no difference despite a slip angle of 40 degrees and full power).
I experience the very same thing on some landings, and the way you describe everything is spot on. Unless we're both crazy, this is some flaky phenomenon going on with the physics modeling in FSX.
What I've found:
!) This only happens with tailwheel type aircraft
2) Seems to be tied to saved flight situation (.flt) files, especially ones that have been saved after the aircraft in question has landed at least once before the save files is initiated.
3) Happens irregardless of crosswind component and loadout
4) Happens when the nose of the aircraft is perfectly aligned with the runway centerline at touchdown
5) There is not enough rudder authority to keep aircraft on the runway after yaw commences
Try starting a flight from the FSX free flight menu. Select the A2A Mustang and then select whatever airport you wish to fly from. Select clear weather. Start up, fly around and land - see if you experience the adverse yaw at all under these circumstances.
And you know what else I'm finding? If I intentionally touchdown with the nose crabbed off to one side, the Mustang will of course start skidding sideways, but I can fairly easily recover using copious amounts of rudder. The landings aren't pretty, but they are salvageable. HOWEVER, as you and I have both experienced, when the mysterious adverse yaw bug appears out of nowhere, it doesn't care how perfect your landings are, the aircraft is simply going to yaw violently off to the side no matter what. So what I'm trying to illustrate is the yaw bug isn't seemingly tied to your landing technique at all; rather, it's completely random.
The super weird thing from my experience is that I can almost always make the yaw bug happen when I used certain saved flight situation files. I promise it's not my imagination, lol.