extended display desktop not working

I have two displays. iMac and Thunderbolt screens.

Until Monterey upgrade, when I used "Extended display", both displays would show a new screen when I moved from desktop to desktop. That is, the displays were linked together in true extension.

Now, I can "Mirror" [which I don't want]. But, when I extend the second display, it actually makes a new desktop in the second screen. While I can move windows between screens, when I use Mission Control to change desktops, only the main desktop changes. This is not a true extension.

Apple support says, this is the way Monterey works.

Does anyone else see this? Do I have a software or hardware problem?

I have given feedback to Apple, if this is a true change.