Another thought - if you meant to ask how to restore a window after minimizing it to the dock with cmd-M (rather than hiding the app with cmd-H), there's an even less well known shortcut: Open the command-tab switcher, select the app you want, then hold down option and release command.

Doesn't handle multiple windows gracefully (if the app has another window not minimized it won't do anything, just switch to that window as normal). But if you have an app with a single window and you've minimized it, this will pop it back up.

Alternatively, you can access the whole dock directly with a keyboard shortcut using ctrl-F3 (add Fn if needed depending on your keyboard setup).

Not sure what to tell you then, since I've never had it not work.

If you tap the command quickly it should happen immediately without even bringing up the app switcher, and if you hold cmd and tab through the app switcher it does the same thing when you let go:

https://streamable.com/pn8rjb

Video recorded on Big Sur beta, but I'm sure this behavior isn't new.