Hi, @DDD ,
I managed to reproduce you issue and found a solution for you. After creating 20k items in my list, the folder I created will not get displayed in the modern experience and the classic experience will not work properly.
As the most common way, we can edit the current view. Add a filter to only display items with a number under 5000. Or we can directly only display folders filter the content type. Only folders will get displayed.
If you still have too many folders, just filter the folder name will be doable.
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