When I select the "selectAll" checkbox, only the quantity number from the "pageSize" property is selected. When I scroll through the grid, the new visible rows are not selected and the previous rows are no longer selected.
Here is an example of a stackblitz:
https://stackblitz.com/edit/angular-tybn6w?file=src%2Fapp%2Fapp.component.ts
of course this works like that. This is the quintessence of virtual scrolling.