I don't understand why this Fillet would fail when it was fine before I changed the length of the edge. See before and after shots attached..
The geometry/features that are used to create the fillet are exactly the same.
Is it possible to post the IPT with the failing Fillet feature here? If it's not possible/appropriate to post the model publicly, you can email it to myself (jake.fowler at autodesk.com) so that we can investigate what's going wrong here.