Dougiesy,
Sorry I can’t solve this for you, however I can at least confirm that I am having either the same or a very similar issue! It’s like the plane has a mind of it’s own while the wheels are in contact with the ground; once I lift off it’s totally normal. And I have the same problem on touchdown at idle throttle in a dead calm, so it’s not wind compensation or normal ‘prop torque’ as others have suggested elsewhere. As soon as the wheels touch down the plane wants to dart left or right in a 90 deg turn. I’ve checked everything like you have and I’ve confirmed no assists are on. It’s more like a ground friction or ground steering bug of some kind.
If I come up with anything I’ll come back and report.
Cheers