Hi Martin, I've tracked down that fillet problem and have it fixed up for the next v3 beta.

It was related to a change back quite a few versions ago in v3 that used a different method to create a closed fillet surface to make them smoother at their closing seam. In this particular case the new fillet surface maker was not refining it until quite enough accuracy that was needed for some of the next fillet operations.

For this particular case right now if you scale it up by 10 times in size you will then be able to get the particular fillets that you need.

- Michael