Explanation
To debug a screen flow that creates contacts, one of the possible causes for a variable missing on the debug screen is that the available for input checkbox was unchecked for that variable. This means that variable cannot be set by external sources such as debug inputs or URL parameters. To fix this issue, check this checkbox for any variable that needs to be set externally. The flow version or field type does not affect variable availability for input. The available for output checkbox only affects whether variables can be passed out of flows or subflows. References:
https://help.salesforce.com/s/articleView?id=sf.flow_ref_variables.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.flow_debugging.htm&type=5