If I understand your video correctly, the behavior you are seeing is this way by design.
There are three operations that are supported by these icons:
1. switch from an incomplete arc with radii to one with no radii
2. perform the reverse of the above
3. switch from either of the above to a complete closed ellipse
I do not think that switching from a complete ellipse to an incomplete arc is supported, probably because the final desired state is not known in this case.
Hi, thank you for the feedback
[I do not think that switching from a complete ellipse to an incomplete arc is supported]
- could it be moved to a wish list?
this way, user could just click get a 1/2 circle
Thank you
Inkscape 0.91 r13725
Mac OSX El Capitan 10.11.5