This problem also concerns my question regarding the multiple grouping inside the JQGrid using the <jqgrid:grid>
tag. But as I later found out here in the changelog of JQGrid, there might be issues with the grouping feature of JQGrid version 3.8.0. I've tried using the scripting format of JQGrid presented here
So in theory, I would download JQGrid 4.0 and use it in the concerned project. The question is how can I use the downloaded JQGrid 4.0 files to my project.