If you use the pager whether in the grid, or on its own and the total number of rows in the underlying data source exactly matches the selected page size, the drop down will be displayed as blank. This will only occur when there is no null "All" option in the list of page sizes. By the looks of it, I would guess that there is code to select "All" whenever the total matches the selected page size option, but it selects nothing when the "All" option isn't in the list. The functionality should select the page size that was selected by the user, not try to reset itself to All if the number of rows equals the page size. The following repl shows the bug:
When you run the repl, change the page size to 5, and then back to 10. You will see that the dropdown selects a blank item. It should stay with 10.