Might be a bug where the preview updates before the document is loaded and doesn't update after it loaded. If you can reproduce, can you check what the filetype shown in the statusbar says before you start editing?

So here is a screenshot from the statusbar after loading. Filetype is ok...

update: I just began typing again and it began working. I resized my sidebar back and forth, then I resized the whole window, then I started typing. I think I reflexively saved (but I do that often enough that I have no memory of it.)

Fingers crossed for the pull request.

Is there a workaround in the meantime?

I am also experiencing this
1.37.1 - Using GTK+ v2.24.32 and GLib v2.66.2 runtime libraries
on Windows 10 64bit

a file foo.markdown is indeed detected as filetype markdown, but the markdown preview tab shows that same

The current document does not have a Markdown filetype.

JFTR, the PR won't help you much: the current version 1.38 and future versions of Geany-Plugins don't include the Markdown plugin any longer.
This is because there are no WebkitGTK packages available in Mingw64 and so we cannot build and distribute the plugin :(.

FYI, FWIW, the plugin is still being offered in the Debian 12.2 repository on my MX Linux distro.
Here's the repo listing as shown in aptitude:

And now, after reading the preceding (March 6, 2022) comment by "eht16", I finally understand why the plugin fails to render previews. Darn it. I was pretty psyched to have Geany double up as a live preview Markdown editor.

p.s. – @codebrainz – It might be worth noting on the website entry (https://plugins.geany.org/markdown.html) that the plugin has been deprecated.

Seems Webkit stuff is broken on all programs using it (Geany, Liferea, …).

I usually succeed by starting Geany (and others) on the command line like so:

WEBKIT_DISABLE_COMPOSITING_MODE=1 geany

Seems Webkit stuff is broken on all programs using it (Geany, Liferea, …).

I usually succeed by starting Geany (and others) on the command line like so:

WEBKIT_DISABLE_COMPOSITING_MODE=1 geany

DYNAMITE ANSWER

Ive looked everywhere for this. What a journey. Needs +100 updoots and put on top of all searches.

Note, as the markdown plugin does not seem to be maintained by its creator any more a kind contributor has provided a fix to move the plugin from webkitgtk to a newer version of webkit2gtk merged 3 months ago, so probably in release 2.1 ("soon" don't ask ;-). Don't know if that will make it available in the windows build as @eht16 comment above notes and I don't know and how long that will keep it working given it effectively has no maintainer.

Don't know if that will make it available in the windows build as @eht16 comment above notes

It is not and will be not available in the Windows builds, this is already the case since Geany 1.38.