I can’t replicate this. I tested using a sample Markdown file found here. When I drag it into a Tinderbox document I see this:
I made some copies of the Markdown note and dragged multiple files into a new Tinderbox TBX document with success. I tried dragging into map view and outline view with equal success.
I don’t think you are, but in case, make sure you drop in the view pane (left side of scree. If you drop into the $Text of a selected note you’ll get something looking like this, and which you don’t want:
How many files is ‘a bunch’? What happens if you drag just one? If you change the file extension of a file(s) from ‘.md’ tto ‘.txt’, does the process work?
In summary, the drag-drop addition on ‘.md’ plain text files should work (as illustrated above): I’m using v9.2.1 on macOS 12.3.1. So the question now is what is different in your case.