I'm not doing QR codes daily, only on some jobs and some trainings. SO I often refer to the right help page.

Where the image field section is precise, and adds visuals of the table itself :

"Add image fields in the data source file
(…)
At the beginning of the data field name, type an “at” symbol (@) to insert text or paths that refer to image files.
The @ symbol is required only in the first line; subsequent lines should include the image paths. Paths, which are case-sensitive, must follow the naming conventions of the operating system in which they’re stored."

The QR Code section gives only little information AND only mentions ONCE the required # sign (which I never remember)

"Though the data entries can be a mix of email, sms, hyperlink, or plain text type. Enter them in the .csv and txt field in the above mentioned format with a # sign on their column name. To generate QR codes, follow these instructions"

Go spot that when in a hurry.

PLUS the data merge panel preview only shows text and images icons, It would really benefit from adding one QR Code line example AND mention those @ and # signs in this section or prerequisite sections.