SPO Views - Filter Pane - How it is working?

Feel like the views that are created in SPO is not behaving consistently. 

We have a large document library with 3 million items. Created multiple views using "Folders" and various "Filters" as recommended https://support.office.com/en-us/article/manage-large-lists-and-libraries-in-sharepoint-b8588dae-9387-48c2-9248-c24122f07c59 and https://support.office.com/en-us/article/Use-filtering-to-modify-a-SharePoint-view-3d8efc52-0808-4731-8f9b-3dfaeacea3d4

Configured the document library (using Metadata Navigation settings) to use "Modifed" as Filter.

We have set of documents that are older than 3 years and counts upto 30. So created a view filtering based on the name of the document. But the view is not returning any rows. 

Also the Filter Pane is also not showing up.

I don't want to add a date filter with [Today]-1095 as it will prevent any new files that's added from showing in the result. Instead added a filter that filters on the name that will return those 30 files. But still it doesn't show up. I am trying to understand whey the result set which is less than 5000 threshold limit doesn't show up? What kind of throttling is happening here?

Also when I first time visit this view, the "Filter" icon shows up. If I click again, it disappears and after that Filter icon never appears. I have to come to "All Documents" view again to get back the filter.

And the frequent updates to how the views functioning is giving hard time and consuming lot of training efforts with the users.