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Caption is partially data driven based on the font metrics. This font has a large descender metric which suggests one or more characters are rendered well below the baseline. Caption centers based on the sum of the ascent and descent which works well for almost all fonts. This exception would benefit from centering based on the trimmed descent of the characters. However, that is problematic if you want to render two different captions with the same font and try to align them side-by-side. We will look into correcting the algorithm so it does not cut off the top of the Q. In the mean-time you will either need to manually position the text or use the Pango layout engine @ http://www.imagemagick.org/Usage/text/#pango, or use a different font.
convert -size 460x70 -background black -fill "#ffffff" -gravity west \
-font /Library/Fonts/ChopinScript.otf caption:"A test with other font" result1.png
convert -size 460x70 -background black -fill "#ffffff" -gravity center \
-font /Library/Fonts/ChopinScript.otf label:"A test with other font" result2.png
So it could be a problem with caption: vs label: centering