There are basically three things that must be done to get a rotary axis toolpath to display correctly in the toolpath view.
1. Designate the rotary axis as "A" No matter if technically the rotary is B or C, it must be called A. Mach will only display A axis correctly.
2. Set the axis of rotation in "Toolpath Configuration" and check "A Rotations Enable"
3. Have the correct value in "Rotation Radius" on the settings page.
If you are, for example engraving, on the outer diameter of a 4" cylinder and the Z axis origin is on that outer circumference, then the correct value would be 2. Z axis origin is offset 2"
If you are doing the same but the Z axis origin is at the centre of rotation, then the correct value would be zero. The value is whatever the Z axis origin is offset from centre of rotation.
In your previous jobs, which never displayed correctly, I guess you had the Z axis origin at the outer diameter but neglected to enter the offset value. This would compress the displayed toolpath.
On this one occasion that the display looked correct, most likely you set the Z axis origin at centre of rotation.