Each time the Brave browser is started it has the same first background. It’s as if there is a list of each background that only goes to a certain tab. For example, my first tab always has the same ocean background, my second tabs always have the same jellyfish background, etc.
It should be a default feature to have a completely random Brave background every time a window/tab is opened. I understand that sometimes there will be duplicates; you might get the same image you already had in your previous tab, but I think that’s still better than what there is now.