FYI @TrippyLighting - every time you publicly reply to a bug report, it reduces the changes that the Fusion 360 team will fix this. You're giving the programmers "ammunition" to argue that they should not correct it, and/or pushing the priority down the resolution queue. Just saying. I'm happy to receive private feedback if you don't want to jeopardize development!
Besides that - thanks. I disagree that it's "garbage" though, mostly because that introduces the problem of trying to decide what IS, or IS NOT, "garbage" in the first place. Splines are just math, whether or not they "loop" (which is detectable [e.g. to reject creation in the first place], and/or correctable by adding extra points [e.g. so input can be always accepted, without mathematical issues creeping in]), so there's no reason Fusion 360 shouldn't properly process all user input without exceptions.
Regarding the "insidedness" of a 3D shape, a fascinating topic in itself - even if this was "garbage", it still should not fail. They probably need to evaluate what "inside" means from more than one place around the shape, and use whatever is the consensus. About 20% of these shapes fail (random internal errors) about 10% are "backwards", and the rest work fine. Getting this about 70% right is an extremely high indicator that this is a programming bug in their patch tool (remember - extrude etc still works fine).
Personally, I blame the cause of this issue on impossible numbers: it gave mathematicians a new curiosity they can use to take math from 1D easily into 2D, but there's no other handy concept outside of the square-root of negative one which extends that concept out to 3D - if they'd never messed with that in the first place, a whole pointless branch of mathematics could have been avoided and replaced with matrices which better extend to this instead... Just my 0.02c ...
