To reproduce:
1. Open the AudioMixer with several groups
2. Try to click and drag on one of the groups
Expected result: You can rearrange them
Actual result: you can't.
Workaround: You can still rearrange them in the Groups view of the Audiomixer

Won't fix because:
This is a feature request that will not be implemented. The channel strips shown in the top part of the window are only a subset of those shown in the full hierarchy view, and therefore only in the full hierarchy you can rearrange them. The Views group lets you make selections for what you want to show in the top view and the order is either alphabetical or as defined by the hierarchy.