Hi Danny, after a bit more investigation your curves are only different in elevation by a very tiny amount, like 1e-12 , they are so close enough to the same that it should be ok to use them as-is, but the curve sorter mechanism was using a too tight tolerance and deciding that they should be extruded separately from one another.

I've tuned this up for the next v3 beta so that the curve sorter will use a more reasonable tolerance for coplanarity detection and that will make your original file work in the next v3 beta. Thanks for posting the example file!

- Michael