It appears that footnotes and columns are not friends of each other.
Setting \setupfootnotes[location=columns] does not place the footnotes in the columns but still at the bottom of the page. Furthermore the \startcolumns..\stopcolumns sequence seems to have an adverse effect on the textheight, because the footnotes can be seen to sometimes intrude in the textarea (or vice versa of course).
Changing the footnote setup to location = page, firstcolumn, lastcolumn, nothing different seen.
Is this a known problem.

I am refraining for the moment of a simple demo, because that is always difficult to achieve with this sort of placement problems.

Hans van der Meer