The same thing happened to me. I realized that the "Title" column that each list automatically creates is a required filed. However, it wasn't a field I used on my form. Since the field was not on my form a error did not appear. Perhaps this is the same issue you are having. To correct this issue follow the steps bellow.
1. Click on your List Settings
2. Click on the "Title" column
3. Click no for "Require that this column contains information:"
4. Click save.
5. Open list form in SharePoint
6. Click on menu (ellipses)
7. and "customize with Power Apps"
8. If you are prompted to override, click yes.
9. Once Power Apps opens the form refresh you data source.
10. Then click PUBLISH.
The reason for opening the form in Power Apps, refreshing the data source, and then pushing (publishing) it back to the SharePoint Site Form is because you made a change in the list (column and or settings) which doesn't automatically update in your site form. Hope this hep!