Hi @ppei,
Do you want to use the radio button to save single line?
What is your really issue?
Cannot show the current select value after save form or can not save value to data source and can not retain the value when you select his item?
If your issue is can not show the current select value after save form, could you please check whether your OnSuccess Property in your Edit Form not set Back();
If your issue is not save it in the data source after save form and not retain the value when you select this item?
I think you can add the radio button into the datacard you want to save,and then you should change the DataCardValue.Default to Rdio.Selected.value and set the Rdio.Default to ThisItem.Column.
You can try following workaround.
I test as next:
Data source:
20190212ratio_to_singleline(my sp list name)
LearningType(single line)

Then PowerApps->Create an app
App Test:
1.In EditForm1:
Add Rdio control and Radio1.Items: ["Approved","Disapproved"]
DataCardValue4.Default: Radio1.Selected.Value
You can also choose to set DataCardValue4.Visible:false.

2.If Issue as first: Please set OnSucces Property Back() in Edit Form.

If Issue as second: Firstly,please set as the AppTest’s step1 and step2, and then set Radio1.Default:ThisItem.LearningType

Setting test:

Result test:

Hope this can be helpful.
Best Regards.
Yumia