Hi NightCabbage, I tried to reproduce your steps but over here I don't seem to have a failure.

Oh wait - it looks like it happens if you do a window select for selecting things, rather than clicking on each one.

Normally with curve fillet on 2 separate curves, it looks at the points that you clicked on to decide which parts of the curves to keep, that's to handle cases like 2 lines that cross each other in an X type shape. If you do a window select it tries to keep the longer piece or something like that, that part seems to be where you're running into a problem.

Can you confirm that if you select the 2 lines by clicking on them instead of doing a window select that the fillet works ok for you?

- Michael