If you press the queue button with tracks in both lists, you can see that these two queues follow each other. The editable queue always plays first.
The problem is having a locked queue at all.
You can't empty it without interrupting playback, and you'll have to use a workaround to do so. You can't reorder it or add or delete songs. After finishing the editable queue, Spotify always continues playback from the locked queue.
The workaround is starting playback at the very last item on a page. The locked queue will then have no further items. Then you can continue by purely using the queue right-click function instead of double-click or enter. This adds items to the editable queue. These tracks can then be reordered and deleted as you wish.
This UI behaviour is, however, not very user-friendly and quite frankly really annoying. It's also very unconventional having two queues in a music player. I've never seen any other software using either a dual-queue system or a locked queue system.