The issue: tapping on a Lemmy link from outside the Summit app doesn't always redirect to the exact destination, but rather to Summit's homepage (subscribed communities feed).

The workaround: in case anyone is annoyed by this problem, while waiting for a fix you could try Lemmy Redirect on F-Droid. After installing it, you set it as default app for Lemmy links (make sure to open its default system settings and enable all the instances to be opened with this app). It perfectly fixed the issue and it's a good workaround while waiting that it is fixed in the Summit app itself.