The alien.top instance block will not be applied correctly after filter is added. I believe part of the problem is that they use a generic name, selfhosted.forum, maybe?

Edit: So, when I Block Instance through the main feed, 'alien.top' is added to the list. Manually adding 'selfhosted.forum' works and the instance is now blocked. That instance is mis-reporting its name and confused Connect.

This is a post example that still appears on my feed: