Hello everyone!

I'm working on a fairly detailed surface model at the moment, and I'm currently doing a shell of the inside by hand.

I've done a bunch of 'Extend Surface' functions with no issue, but this one is playing up for some reason. The preview (below) looks good...

But, the result has big triangles missing (below)...

Does anyone know what's going on here? It seems like a fairly fundamental error, so I've probably got a setting wrong.

Any help is much appreciated, cheers!