Does a rebuild of the database fix this? Maybe the index is not up-to-date, the handling of Markdown documents has been improved many times during the last years.
No, that was the first thing I tried before open this thread 
How do you actually change the documents if the rebuild didn’t fix it? An example document would of course be useful.
I open all documents in edit mode add a linebreak at the end, CTRL-S and go back to the View mode.
The files are normal md documents with the following information in the header (before the first # H1).
Author: Thomas Krampe
Title: This is the Title
Subject: Some subject
Copyright: 2021 by Thomas Krampe
Organization: MyCTX
Keywords: Wordpress,AppointmentHourBooking,Plugin,Language
<script src="x-devonthink-item://E2554DA1-4B5D-4900-8ACC-EFCEBEA58875"></script>
<link href="x-devonthink-item://4011138A-73F7-4475-947B-15AAC859E2BE" rel="stylesheet">
What operating system are you running?
Your example document works actually fine over here, both a smart group and List view work as expected. Does exporting such a file to the desktop and importing it again also fix the issue?
12.4
Well I had this problem with approx. 120 documents and the work around open, change, save is done. If I create a new document from scratch now (based on my md template) everything work as expected. So no issue any more. Thx. for your thoughts here.