Tony, thank you so much for taking the time to test this. I am using jqGrid 3.5.3 (all modules) + jQuery 1.3.2.
I use a remote data source (Oracle) and I don't use addRowData.
I will recheck my installation again, hope this is only my case.
Here's a screenshot of my issue:

P.S. I have found the negative numbers respond to the rowNum. So if you set rowNum to 100, you'll get -99, -98, -97, and so on.