I'm on lemmy.ca and I get the same thing. Doesn't matter if I'm using an app or desktop. I made an account on lemmy.world and the same happens.

But, the subscriptions are working and appearing in my main feed, so I assume it's just a bug that'll get fixed eventually.

Lemmy.ml is overloaded and not running correctly. Your subscription request is probably timing out but your server will still attempt to pull posts and comments.

I have this issue only with instances from lemmy.ml

I think it has to do with it being underpowered

It seems to be a visual bug. Click on the "Subscription pending" button and it will go away.