This type of error usually occurs if another user modifies the source record before the current user clicks the save button. I think it's worth checking your form and process to see how this might be happening. Could someone be modifying the record outside of PowerApps?
If you want to force your save button to win the conflict, you can do this by refreshing your data source prior to calling SubmitForm.
Hello, i m getting the same issue. I put refresh(datasource) before i submit and it still does not work. It will update the item the first time, but it the user wants to update again in the same session it Etag mismatches. Users should be able to edit items as many times as they like, what is the fix for this ?