http://isdb4l77sjqoy2qq7ipum6x3at6hyn3jmxfx4zdhc72ufbmuq4ilwkqd.onion/blog/20231125-simplex-chat-v5-4-link-mobile-desktop-quantum-resistant-better-groups.html
Previously, the problem was that until a new member connects directly with each existing group member, they did not see each other messages in the group. The problem is explained in detail in this video at 18:23. With v5.4, the admin who added members to the group forwards messages to and from the new members until they connect to the existing members.