Issue on SharePoint List Grouping & Filter, not all items included?

Hi All,

Recently, we migration around 15000 items into a SharePoint list, however we got a issue on the grouping & filter, we created a view without any filtering, but only group by "Document Category", but the view only contains 210 items:

Base on the above result, we add a filter in the filter pane, then got the following result:

as you can see, many items can be showed in the list

How can I solve the issue?