It works fine here and on all our tested websites. I recommend to change the theme and test it again.

Also leave some screenshot of the Dashboard > Settings > Discussions admin page, the part of avatars (scroll down). What type of avatars you've enabled in this settings?

Now I made another 1 site with a standard theme. In discussions, avatars change outwardly, but in practice they remain standard. Previously, the problem was with the fact that the avatars did not change on the forum. Apparently, she moved from the forum to Wordpress.

Try to change it to something like this:

Look. Could it be that the avatars replace only the picture in the discussions?

You can test it here: https://wpforo.com/test/#comment-66

We use wpDiscuz plugin: https://wordpress.org/plugins/wpdiscuz/

Try to install this plugin.