I used the YASim PA18.
At both 0 and 500 ft, I manually created a 5kt crosswind at 160 degrees.
Here I was established on Nav1 CDI course, and the plane was crabbing about right: the arctan of 5/80 is 3.576, which is about what the plane is crabbing.
Right before touchdown I unchecked Pitch/Altitude Hold, left Heading Control engaged, and manually flared. So why did the plane want to depart the runway to the right
The bank is the result of the autopilot trying to keep it on the runway.
The bank was the result of the autopilot trying to use its normal method of correcting headings, which is incorrect for crosswind landings, where you would want right aileron, which creates adverse yaw; and which combined with left rudder, would allow the plane to be steered left.
But that isn't the point. Assume the plane was an old fashioned Ercoupe with castering main wheels. Since the correct crab was well established and the wings level, it would have initially been traveling
straight down the runway after touchdown, while still maintaining its crab. There would have been no turning moment, and further control input would have been unnecessary.
The PA18 isn't an Ercoupe, though still tricycle geared. However, I think its momentum would have initially been straight down the runway at touchdown, and it should not have immediately turned right.
Kind regards