Hello - `FlowAlongSrf looks at the UV of the base and target surfaces - some surfaces, while they look simple enough, do not have evenly distributed UVs - Cylinders are among these. If you rebuild that to degree 5 in U with 24 control points in U and degree 1 in V and 2 control points, the result will be very very close to a true cylinder and have evenly distributed parameters.
Another approach in this caase is to UnrollSrfUV the target surface and use that unrolled surface as the base surface.
-Pascal