Classic SharePoint features appearing on a Modern Site

This can happen even on a modern site in Microsoft SharePoint, and it does not always mean the site has switched back to classic mode. In some situations SharePoint automatically loads classic components for compatibility.

One common reason is unsupported features or customizations. If a list or library contains features that are not supported in the modern experience, SharePoint may automatically open the classic interface.

Another reason can be list or library settings. Some lists may still be configured to open in the classic experience. You can review this in List Settings and then check Advanced Settings to see which experience is selected.

Older site configurations can also cause this behavior. Sites that were originally created with classic templates sometimes still contain legacy features or workflows that trigger classic pages.

Custom scripts, web parts, or third-party integrations may also force SharePoint to display the classic interface.

A good first step is to review list settings, check site features, and remove unsupported customizations if possible. After that, make sure the list experience is set to the modern experience.

Hi RMDUser1​ 

in this documentation you can see, how you set the modern experience in your library/list:

https://support.microsoft.com/office/switch-the-default-experience-for-lists-and-libraries-between-new-or-classic-66dac24b-4177-4775-bf50-3d267318caa9?WT.mc_id=DX-MVP-5004845 

And in this, on bottom, you can see the features that automatically switch to the classic experience:

https://support.microsoft.com/office/differences-between-modern-and-classic-experiences-for-lists-and-libraries-30e1aab0-a5cc-4363-b7f2-09e2ae07d4dc?WT.mc_id=DX-MVP-5004845 

Check the articles for a modern experience.

Best, Dave