You've surely been there:
In your SharePoint Online environment, you have set-up the modern experience to apply to all the sites and lists.
You create a document library and, naturally expect to see the new experience applied. Still you will be redirected to the classic experience.
Why?
The answer is here :Differences between the new and classic experiences for lists and libraries
Still going through the document, you cannot find a specific reason so, here's how to read this:
Point 1: Features available only in the classic experience : This basically means that if you expect one of the listed features in Modern Experience, you will simply NOT have them (for example, location based views will NOT work in Modern).
Point 2: Features available only in the classic experience : Customizations
List templates other than
- Document Library,
- Custom List,
- Announcements List,
- Links List,
- Picture Library,
- Asset Library,
- Promoted Links,
- Form Library,
- Wiki Page Library,
- Asset Library,
- Promoted Links,
- Wiki Page Library (Site Pages/Pages).
Which basically reads : List templates other than ... Are available only in classic experience (or the other way around, the list templates other than... do not support modern experience)
Now what happens if I do have one of the features named in the first part of the article ?
The answer reads : you will be automatically redirected to the list's classic experience (described in part 2).
Point 1:Features that cause the new experience to automatically switch to the classic experience
Example, you add one publishing column to your custom list, you get the classic experience , even if before you had your list working fine in modern.
Point 2:Features that cause the new experience to automatically switch to the classic experience : Customizations
List templates other than ...(the list above)
Which reads: if you create a list different than the supported template(above), you will get redirected to the classic experience.
Sure, you can specify in the document library setting under Advanced->Experience to have the lists that do not support the modern experience to always use the new experience.
But this at the same time will mean that the list will render as a simple list and the items will be simple list items.
Let's take for example a task list.
By default you will get in the tasks list default view a view of the entries, and a view on the timeline of the tasks.
Set the list to new experience and the list will render as a simple list.
Click any on the items in the list and the items will be downloaded instead of opening in the custom edit form associated with the list.
Conclusion:
Forcing the modern experience on a list that does not support it is ... well... not supported.
Use the lists as they are. Modern in modern and classic in classic.