The QuarkXPress document converter only converts legacy QuarkXPress 3.x- 6.x documents to QuarkXPress 9.1 but does not upgrade its text flow version. When a user opens such a converted document in QuarkXPress 2022, then the text flow version will get upgraded and it will become equal to the document version. Hence, the text will reflow. The same behavior occurs if the user updates the text flow version of a legacy version document in QuarkXPress 9.x, (by opening the document by pressing Option/Alt key) and then saving it.
The Legacy document converter only converts legacy QuarkXPress 3.x-6.x documents to QuarkXPress 9.1 but does not upgrade its text flow version. When a user opens such a converted document in QuarkXPress 2022 and later versions, then the text flow version will get upgraded and it will become equal to the document version. Hence, the text will reflow.