Let's parse this:
"clicking on a link to a post on another #Fediverse instance"
"opens the link in a new tab"
"instead of opening the post in my instance"
You click on a link to a post that's not on the instance you're homed on
Your web browser (right?) opens the link in a new tab
The new tab contains content that is on a different instance than yours
Have I got this right?
Sounds like everything is working exactly as it's supposed to
It has nothing to do with the "maturity" of Mastodon
It has to do with how http links to other web sites than the one you're immediately looking at really work
Been that way for decades
Sorry