macOS Big Sur - Set quick look provider for specific file types?
AFAIK, the default app for opening documents and the QuickLook generator for previewing them are two completely separate things.
If you have a QuickLook generator file for the .md format, then that will be used by the QL manager.
The Terminal command qlmanage provides a way to get the status of the QL manager, and to configure it in a limited way.
&gt; qlmanage -m | grep &quot;md&quot;
  com.unknown.md -&gt; /Users/Ben/Library/QuickLook/QLMarkdown.qlgenerator (1.3.3)

You can force which QL generator is used with
qlmanage -g /path/to/generator/