Hi Michael, Good to know - you know.

I just had the same experience today... After I Boolean Differenced a small object from an even larger one I went to Fillet a bunch of segments that were all on connected surfaces that had funny angles to each other - it worked, but the entire solid broke down into separate once-joined surfaces.
Okay, it was a joined surface that I had to reconstruct anyways and then no problem from the Fillet.