When you follow someone or someone follows you, you can't see her/his avatar if he/she is a remote user. However, with some servers (mastodon.social, status.fsf.org, gnusocial.no) it works but with mostly others it doesn't (so it's a heisenbug). If you run an installation of GNUsocial with PHP 7.0 it works OK.

I used Debian 9.5

PD: If this project is being maintained I guess this would be fixed ;)