I am logged in lemy.lol. I am not logged in programming.dev. I go to community in programming.dev. I press Subscribe, I enter lemy.lol. I press "Fetch Community". It redirects me to login page of lemy.lol. I just refresh the page without logging in and I am redirected to "lemy.lol" homepage and I am logged in.
Edit: I tested it with other instances where I have account, the result is the same, it's not just lemy.lol
I usually use the Brave browser, but I used Thorium browser for the screenshots. I tried with adblocker on and off, I think there is no difference
I tried with fresh installed Chromium too. This problem is not specific to our instance. I guess I will open a PR for this.