I notice if a .markdown file is contained inside a DEVONthink database package, then Marked does not sense changes to the file and doesn't update the preview. DEVONthink databases have a folder structure where documents are stored in their native form. In this case, I've been storing .markdown files in DEVONthink, opening them from their to edit in Composer, and using the Composer option to use Marked for previews. The workaround is to command-w the preview window before telling Composer to preview again. No big deal.

(I've not noticed this issue with other document-types stored in DEVONthink and edited in their native editor.)