I've recently come to the understanding that mac users have been encountering a few problems with the Game Detection Feature as well as the overlay feature. This is due to the fact that discord DOES NOT in fact support 3rd Party or Modded Applications such as ROBLOX.
What this means is that Discord doesn't automatically detect you playing a game such as ROBLOX and you would have to remove it from the "Added Games" section in the Game Activity part of settings and then re-add ROBLOX while playing it which will then enable Discord overlay. You would have to repeat these steps every time you open the ROBLOX app, support from my fellow MAC users would be much appreciated!
Upvoting this post can greatly contribute to their decision making; if you're a devastated mac user, much like myself, please upvote this post!