@_elena I made a test post using your comment form with the same issue, so this is not an ActivityPub only issue.
Maybe your DB is not setup properly?!?
You have to use `utf8mb4` as charset and `utf8mb4_unicode_ci` as collation.
https://make.wordpress.org/core/2015/04/02/omg-emoji-%F0%9F%98%8E/