forceFullDesktopBar

Prior to MacOS 10.11, Mission Control was good: you would swipe up with four fingers and it would show you a preview of all of your spaces. Then in 10.11, for no discernable reason, they changed it to suck: rather than showing you a preview, the bar just says "Desktop 1", "Desktop 2", etc until you mouse over it; the practical effect is that using spaces is disorienting and requires memorization. Some third-party software pretends to restore this functionality, but they do it by repositioning the mouse to simulate a hover, which introduces a delay and doesn't integrate correctly with the animation. Someone wrote a patch that works by disabling SIP and injecting code (https://github.com/briankendall/forceFullDesktopBar), but eventually stopped maintaining it. A decade later, I doubt anyone at Apple remembers that this bit of user interface

They should transfer over to your main display (at least that's what happens for me). This utility might help you see what's going on: https://github.com/Jaysce/Spaceman. And while we're on Spaces, this utility forces mission control to show full thumbnails of all spaces by default: https://github.com/briankendall/forceFullDesktopBar