Hello, I also experienced this problem yesterday on a project that I am currently working on, and darn near pulled all my hair out wondering what the problem was. and cost me much sleep last night Me and a colleague have narrowed this down to a bug with the Modern Date / Time Control.
We have learned that these are the steps to get around it (not optimal) and hope Microsoft will fix this soon.
1) When attaching my new SharePoint list, anytime you have a data source with a Date/Time, it first needs to be created as a Single line of text.
2) Then connect the data source in Power App, and refresh to get the new schema.
3) Insert a new Modern Form control to the datasource.
4) Save, and Exit the Power App.
5) Make the field change in your SharePoint List to change it to Date / Time.
6) Open the Power App, remove the Date Field you previous created as Single line of text.
7) Once again, save and exit the Power App.
😎Open the Power App back up again, in your form, remove the Date Field that was a Single Line of Text, and re-add the Field.. which will now be a Date and it will work.
Hope that helps someone else out here!
#TLDR - Create any date fields you have first as Single Line of Text, then after connecting in the Power App, change the SharePoint column to Date format, and then update your data card on the form. This was also a problem with the new Table experience as well. Tested in two different tenants and environments, same issue!
