The Grid PageSizes DropDownList will not select a new value if it the current value is not visible in the open dropdown. This can happen if there are too many items in the DropDownList and the user has scrolled the PageSizes dropdown.
The problem occurs only when Navigable="true".
Here is a REPL test page
The easiest workaround is to fewer items in the PageSizes DropDownList, so that there is no need for scrolling.
I tried as you suggested and reduced the number of items. It works better but yet not perfect.
Initially is 20 the pagesize value when opening the form. If selecting pagesize 5 it gives an empty selection. I should click on 5 again to see that pagesize 5 is what I've selected.