I have made a test on my side (Within published app in web and PowerApps mobile app) and don't have the issue that you mentioned. I could not reproduce your issue on my side currently.
Please check if the version of your PowerApps Studio for Windows or PowerApps app within Mobile Device is 3.18061.12. There is something wrong with the previous version (3.18054.16) of PowerApps, the user @KK89 has faced similar issue with you and the PowerApps Staff @LanceDelano has made a response to it within the following thread:
In addition, I agree with your thought almost. As an alternative solution, you could consider take a try to create a new date/time field in the your SP list, re-engine your app pointing to the new field, then modify all formula related to the date field.
The version of power apps is not the issue. Mine has been updated and the problem still exists.
It is also not the solution to recreate new fields in the list or to completely recreate the list. While this has worked for a short time, before I had finished recreating the list the problem returned.
There is currently no fix for this. I have a case open with microsoft support. They have looked at my issue and have not yet been able to provide me with a solution. Nothing mentioned in this thread will help solve this problem.
We have customers who have reported that the fix is active. Make sure you have the right version, open your app, make a small edit and save and republish. Pleasse let us know if this does or does not work.
Still waiting for a solution (Same problem with SQL).
The SubmitForm won't save the Date Datacards. I have the version 3.18081.22.
I find this weird that it detects changes to the field (Form.Unsaved) but can't save it. Even after i SubmitForm, the Form.Unsaved is still unchanged.