I am using DF Pro 10.0 Beta 22 on Windows 10 Version 21H2 Build 19044.2728.
When I click the Show Desktop button on the Windows taskbar or the DF taskbar, it minimizes all the open windows on both monitors, and the DF Taskbar on the second monitor is also hidden. I don't think it used to do that (I mainly hit the button by accident).
Is this intentional?

Yes, I have my taskbars all set to the top of the monitors.

I never noticed this before, but can confirm ... at least in one instance on my system (running DF beta 25). I often have a remote desktop window open on my second monitor, and it's in full-screen mode. As such, the RDP window completely takes over monitor #2, showing its own taskbar, etc. When that RDP window is open and I click on Show Desktop, all windows are hidden, including the RDP window, as they should, but the DF taskbar on monitor #2 does not appear (just as the OP described).

What's unique here is that the DF taskbar is not shown anyway when the RDP window is displayed full-screen, but I guess I'd expect it to reappear after clicking on Show Desktop to hide all open windows.

I did try maximizing other windows on monitor #2, and the DF taskbar remains after clicking on Show Desktop, as it should.

Just for grins, I moved the maximized RDP window to monitor #1, which has the standard Windows taskbar. It hides the Windows taskbar, as it normally does, but when clicking on Show Desktop the correct taskbars appear on both monitor. So the reported behavior seems to be unique to monitors other than #1 (or, probably, only monitors that have a DF taskbar rather than the standard Windows taskbar), and specifically when a window on that monitor has taken over the entire screen.

I haven't found another app that when maximized to full-screen takes over the entire screen (and hiding the taskbar on that monitor) like RDP does, but there may be others.

[EDIT: my taskbars are all on the bottom]

I'm having the same issue.
I have a dual monitor setup with the taskbar for the left monitor set to right and the taskbar for the right monitor set to left. Hoping this gets fixed soon!

dump, same isse, second monitor is in portrait mode and left aligned and show desktop from alt+tab or windows native show desktop function both hides the DF taskbar on second monitor

Just checked and the issue still exists after more than a year.