We would suggest to create a custom field (with a Standard storage) instead of using the introtext (custom syntax) one. Create you "my_text" field and use "my_text" within your CSV file.
We may try to test/fix the Importer behaviour with the "introtext" column, but it's really been a while (many many years!) that we don't use the "introtext" to store custom values/fields. No benefits doing it.
Standard or JSON is the way to go for all your custom data.