I don’t know the answer because I can’t speak for Airtable, but support for rich-text in CSV is problematic for many reasons. CSV, after all, is a formatting standard that allows us to create data grids that are composed of plain text. It’s possible to include Makdown in CSV, but such tags would not be interpreted by Airtable long string fields even if enabled for markdown support - instead, the content would be interpreted literally. I haven’t performed any tests to be certain about this, but I believe this is exactly how Markdown tags would be interpreted in a CSV upload.
Remedy
I’ve done a number of integrations involving the transformation of rich-text formatting tags into Airtable rich-text fields with success. A good [simple] example is building automatic links without displaying the underlying URL - like this.
To achieve this requires an API transformation that is instrumented to craft the appropriate markdown tags and which could be done with a Script Block or an external process. Given that you have sizeable numbers of records, the latter is probably the right approach.
When you import the CSV file the Rich Text field is not available (grayed out) to import data. Records are being created with empty Long Text/Rich Text fields, despite data being available in input file. See the image below.
