There is an interaction in the placement of float captions when their width is set and background coloring switched.
Minimal example included here, the jpeg is arbitrary of course. Output shown.
Setting the width of the caption effectively shifts the caption to the left.
See the change between the last two floats where [background=] changes the game.

I wonder, is this intended behaviour? Or should one expect something different?

\starttext

Testing figure captions with \tex{setupframed[background=]}\blank

\setupframed[background=color,backgroundcolor=yellow]
\placefigure[][]{\tex{setupframed[background=color]}}{\externalfigure[test.jpg]}

Add \tex{setupcaption[align=middle,width=\makeupwidth]} as setting here:\par
\setupcaption[align=middle,width=\makeupwidth]
\setupframed[background=color,backgroundcolor=yellow]
\placefigure[][]{\type{\setupcaption[align=middle,width=\makeupwidth]}}{\externalfigure[test.jpg]}

\setupframed[background=,backgroundcolor=yellow]
\placefigure[][]{\tex{setupframed[background=]}}{\externalfigure[test.jpg]}

\stoptext

dr. Hans van der Meer