I've made a simple blazor repl demo here: https://blazorrepl.telerik.com/mmlYatun39gTFEgO21
As you can see, TotalCount is 10, and the current PageSize is also 10. When these values match, the value in the "items per page" dropdown isn't present. Select any other value, and it will be present (for 5, 20, 40). Come back to 10, and again, the value disappears.
Image examples, with items per page set to 10, then 5: