I'm wondering if you are going to the three-dot menu and selecting Subscribe. It immediately changes to Unsubscribe since you are then subscribed. May or may not be it. Just trying to help.

For me, I noticed that i have to logout and then log back in for my subscription list to refresh and reflect a new subscription. This is the case both on mobile and with Connect. This is on Lemmy.ca, with both a local community and another one from another instance. So it doesn't seem Connect specific. Not sure if you're having a similar problem or another entire since you mentioned being immediately unsubscribed.