Hi,
I've seen this in Excel, I've never seen it in SharePoint, so weird.
Is the Text Input set to Text or Number Input (1st page of properties)?
Let me add this though
If you want to edit the Form, you have to pick the given DataCard, and unlock it, via its properties on the Right.
Then you can modify the Card, to be another control, or you can try to fix the current issue, again, checking
the properties and just make sure its not set to Number versus string.
However, once you unlock the card, you can delete that control and Add the date picker.
then in the Default property of the Picker, do ThisItem.DateFieldnameHere
