Some users have reported that Microsoft is replacing the default Modern List view in SharePoint on Microsoft 365 (SharePoint online) with Microsoft Lists.
I haven’t seen this yet on my SharePoint online developer site, but it is being rolled out and should be completed by the end of September.
Microsoft Lists has a similar look to the Modern List view, but it causes the Classic DFFS to fail to load because Microsoft no longer takes into account the established method of using ContentType.NewFormUrl, ContentType.DisplayFormUrl and ContentType.EditFormUrl property to change the default forms for the list.
If you experience this issue in your site you can fix it by going to Site settings and under Look and feel select Navigation elements and under Lists in Sites toggle Show header and navigation on.
You can read more details and find a PowerShell cmdlet to set the ListsShowHeaderAndNavigation property here: https://m365admin.handsontek.net/microsoft-lists-user-experience-update/
Let me know if you have any questions or comments in the comment section below.