Plus icon appearing incorrectly in grid

Question

For my grid I have the following configuration:

        subGrid: true,
        subGridOptions: {
            // configure the icons from theme rolloer
            plusicon: "fa fa-plus",
            minusicon: "fa fa-minus",
            openicon: "fa fa-expand"
        },

When the grid is displayed, the wrong icon is showing up:

This is the generated html:

How do I get the normal +/- icons to show up?