This script fixes the problem: https://spjsblog.com/2020/07/30/fix-classic-form-fallback-on-modern-lists/
I have gotten a few reports from users of SharePoint Online that have opted in for “First release”.
When using modern experience for list views and you click New, Open or Edit to add new or view / edit an existing list item, the auto detection of a customized form stopped working.
This customization could be either a web part added to the form (like for DFFS), or a JSLink customization added to a field.
Starting yesterday, instead of falling back to the classic form like it has done for many years, it now shows the default modern form in the slide-out side panel.
I have opened a ticked with Microsoft and have been in contact with them to describe the issue in detail.
At the moment I don’t have any other workaround than to switch back to classic for the affected lists, but I’ll update this post as soon as I know more.
Sorry for the inconvenience,