The “Fillet Tracks” tool is just meant to be a simple tool for the time being (better than nothing at all). I think what you suggest might be better achieved with an interactive filleting tool where you could drag the corner of two adjacent straight segments to create or ammend a fillet, like below:
We are now in v6 feature freeze so only bug fixes are permitted in addition to a very limited set of features that are still in development. I’ve been trying to code the above feature but am struggling with it right now so not sure I can make it in time for v6 and if I did it might not be accepted.
Also @Scott_Cox if you do find any other DRC errors either Windows or Linux please let us know as it might be a bug either in DRC or the Fillet Tracks tool.