Spent the tiniest bit of time on this - it is incredibly common, but the implmentation isn't consistent. Some games have X+Y as a on/off switch, others force you to hold the buttons, and there were definitely cases on the Saturn where you couldn't "undo" a clean pause without unpausing first, so I imagine the Dreamcast is similar. And sometimes pause menus still work, even though you can't see them. There are some obvious reasons why you wouldn't want to mandate this feature - pausing in a game of Puyo Puyo Fever might give you an advantage, for example, so they hide the playfield (as they do in all Puyo Puyo games worth their salt). GameFAQs only lists a couple dozen games, I'm certain there are hundreds of Japanese-only titles that have never been tested.