Hey Adam,
I converted it to an editable spline, otherwise it wouldn’t allow the use of the project tool.
I copied both elements into a new project and still wouldn’t work, so I created a new cylinder, made it editable and deformed it to match the curvature of the original object. Then I hid the original and projected the spline onto the new cylinder; then worked correctly. I used xy plane projection. After that I deleted the cylinder and unhid my original shape.
I’m still not sure what was wrong with the first object, but the problem laid with that, not the spline, maybe because it was triangulated? Very weird.
Thanks for taking the time to look into it.
D.