I just found out that Debian Stretch prefers ip6 over ip4 by default (I guess they changed that after Jessie). In other words, it tries to lookup #ip6 first and if not existing it uses #ip4. For Mastodon this is important, cause instances that configured ip6 incorrectly (like being inaccessible from the outside) have problems accepting follower requests. To fix this, follow the advise here (for Debian, use Ubuntu how-to):

https://community.rackspace.com/products/f/25/t/5110

#mastoadmins