Please can someone explain why the column before the floating figure is empty?

I thought that the figure would float through the text to the first location with sufficient space. That appears not te be the case: the input behind the figure does not typeset before it although there is ample space for the text that now follows the picture.

The code is roughly:
\setupcolumns[n=2]
\startcolumns
<several paragraphs>
\placefigure[][figuur:zw]{Street.}
{\externalfigure[zw][width=0.8\textwidth]}
\par
<small paragraph following picture>
<several paragraphs extending to next page>
\stopcolumns

How to get
Hans van der Meer


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