I should also explain the design choice here.
The reason why it's not done automatically is to preserve the intent of the link. Eg. let's say you tap on a link to a post P and that post has comments A and B on it, but comment A and B only appears on lemmy.world. Then if the instance switch occurred automatically, you will only see the post without comments A and B. This can change the intent of the link.
Eg. imagine the following scenario. A user posts "how do I unclog my drain", another user comments "I found this post with some helpful answers". The first user then taps the link but the link auto opens in that user's instance which just happens to not have any of the comments. The user is then confused how the post is helpful.
I can add a feature to auto-open link in the current account's instance but I will make it default to off.