This is a common problem that I've just have accepted, but it drives me a little crazy when there's several changes to label or symbology design that I am experimenting with.
Whenever I apply any change to these panes (not exclusive to symbology or label panes), the pane seems to close and I am forced to right click the layer in 'Contents' to retrieve the pane once again.
Is this something that is controlled via Preferences Menu or Settings Configuration? Or a bug?