Presently each attempt to follow a link which opens within a recognized app (today's example, a link to Twitter from Safari or Signal) will instead lead to the App Store, which was an appropriate step for an app which I did not already have loaded, but is improper process for an app I already have onboard, configured, and properly signed in.
Open In App failure
If the question is about an app (Safari, Signal) and not your own code, you'd better contact Apple Support Community: https://discussions.apple.com/welcome