Not a bug, although it may be unexpected. It's a grid snapping thing. Try:
\def\example{%
Lorem ipsum. Lorem ipsum. Lorem ipsum.
Lorem ipsum. Lorem ipsum. Lorem ipsum.
Lorem ipsum. Lorem ipsum. Lorem ipsum.
Lorem ipsum. Lorem ipsum. Lorem ipsum.
Lorem ipsum. Lorem ipsum.
Dear Hans and ConTeXt developers,
When you compile the following code with ConTeXt ver: 2021.10.28 10:19, you
will see that:
* in 1-column text the top spacing of the circle is normal,
* in 2-column text, the top spacing of the circle is too large.
Here is the link to the output PDF: