When creating a "Single Line Text" type variable from an existing "Reference" type variable, the system retains the previous reference from the original field, even though the newly created field is a different type.
Instead of converting an existing reference variable to "Single Line Text", create a new "Single Line Text" variable from scratch.
This behaviour is by design for the insert and stay functionality, which is copy the data from the form into the new record. A possible enhancement is under review. To receive notifications when more information is available, you can subscribe to this article by clicking the Subscribe button at the top right of the page.