That looks more like an issue with the 'multiple outputs' plugin. You may need to reach out to the plugin author about it, so if the plugin has a certain minimum width it will always claim that minimum width.
Some panels DO forcibly resize on certain contexts though (the Status dock is a great example); if they 'need' more width, even just for a moment while they're initializing, it can scramble up the UI layout pretty badly.
It doesn't fix your problem, but as a workaround I'd suggest trying to move the Scene Transitions dock into the empty space under your Controls dock (Start Streaming/Start Recording/etc buttons). Having the extra space available there, or possibly slimming down the scenes and sources lists could allow margin for that (temporary?) size readjustment by the plugin.