jQuery Grid Plugin - jqGrid

I'm using version 4.1.2.

I have a grid with the option scroll: true. The documentation says, "When set to true the grid will always hold all the items from the start through to the latest point ever visited." I'm seeing different behavior. After the initial load, when you scroll down, the next batch is fetched (as expected). However, some of the rows at the top are lost, and when you scroll back up, those rows are refetched from the server.

We need the behavior described in the documentation -- that is only new rows fetched upon scrolling down and previously fetched rows forever retained. Is this a bug? Is there some other setting that required to make it behave as documented?  Since I'm also using a subgrid, could that interfear with the desired behavior?

Thanks,

Greg