Hello, I'm not sure if this is an issue or not but when interpolating between two curved paths, with the same number of anchor points, there is a random scale and warp transformation in mid interpolation point. Are curved lines too much for interpolating?

I guess that both lines need to have the same direction. Try mirroring one of them. It might works.

ok thank you, I'll try

I'd try reversing one path before running the extension.

Thank you Javier and Tyler, mirroring one of the paths worked very well! I don't understand the direction being different but it works...

 

The first node of the interpolated path lies between the first node of path A and the first node of path B. Reversing path B moves the first node to the other end.

I see, that would explain the scale of the middle interpolated path. thank you Paddy.