The one that always gets me is how oddly forgotten the MMC stuff is:

The Disk Management UI's bottom pane, for whatever reason, has never worked with scroll wheels. It still gleefully ignores scroll wheels.

The Hyper-V Manager .. a new thing .. has a broken UI as well. It's "Actions" pane size is based off the screen-width, but the default size of the window is based off the screen height, so on ultra-wide screen monitors, a pane that should take up about a 1/3 of the window, ends up covering about 2/3 of the window.

I also love how even the Action menu is a hold over from some older UX ideas. There are two ways to bring up the help dialog now in all of the MMC apps, either with the Help menu, or via the Action menu.

But! At least they got enough attention to use Edge instead of the default browser if you try to open the "TechCenter Website" (a website that hasn't had that name in 8 years or so)