I have a power Apps form based on a SharePoint list. One of the fields is a choice field. Every time I save the form, the form is resetting to a value that was once set as a default value in the SharePoint list choice field settings. The value is 1/5/2019. This value no longer exists in the dropdown nor I have the default value set in the Choice field.
How can I fix this problem? Is this a Bug in Power Apps?
It seems that I cannot reproduce your issue. I have set a default value for a Choice column in SharePoint list, created an app for this list, save the app and close it. Then I removed the default value in SharePoint list column setting. Open the app again, (refresh the data source), I deleted this data card in PowerApps. Finally readded this field as a data card, and the Forms work just fine both in play mode and edit mode.
Please try to follow the steps above and check if the issue could be resolved.
If the issue persists, please select this data card, and then check the Default property of this this data card. Then select the dropdown control inside this data card, and then check the DefaultSelectedItems property. Paste these 2 formulas here.