Size difference in middle of interpolation

Hi, I have something I don’t understand, but is probably logical to those who know the Glyphs app well.
I have and axis (weight) with two masters. With a circle on one (Regular), and the same circle on the second (Bold). When I rotate the circle 90 deg on the second master, the circle changes size in the middle of the interpolation.

Can somebody please explain why this happens and how I can fix this?

Thank you.

This is how linear interpolation works. It is not interpolating the rotation but each point. Activate “Show Compatibility” (in the View menu) and select all nodes. The blue lines show where the nodes move during interpolation.

Yes, of course, very logical now that I think of it. Is there a way make this non linear, in a curve for example?
Or is this really complicated?

That is not possible in the current system.

As a workaround you can use several brace layers: