From what I can tell, this is a scheme issue. Right now, Roblox uses the URL scheme:
robloxmobile://placeID=************
(Example: robloxmobile://placeID=3540454961)
This URL scheme causes a number of problems including:
You have already joined this place on another device
This one is hard to repo, I’ll post a screenshot if I can get it to happen again. It generally occurs whenever you try to join the game from the web multiple times over the span of a few minutes.
And the infamous opening of the App Store
I go into much more detail here:
Roblox website on Safari opens both Roblox and the App Store
However, I believe that a solution could be to use the URL scheme roblox://placeId (e.g. roblox://3540454961) I have used this in QR codes and manually through Safari too, and it works fine every time.
Wondering what a URL scheme is?
A URL scheme (in mobile development terms) is like a URL but for an app. It serves as a bridge between the website and the app. Apple has a pretty good overview here: Apple Developer Documentation