I've created a sample REPL to demonstrate the issue:
https://blazorrepl.telerik.com/mHaSmsvW31IZ0rar47
In Grid.OnRead event handler, if you set args.Total to an amount > 559240, then the Grid will only scroll to 559240. In my example, I used 1,000,000 for my virtual data source size.
If possible, use a smaller row height and override the theme to apply smaller vertical cell paddings. If that's not enough to reach the last row, you will have to apply some default filtering, so that the total number of items (and resulting scrollable height) is smaller.