Hello fabian1313,
Thank you for using the Apple Support Communities. We understand your group messages seem to be splitting up into separate messages, and you've done quite a bit of troubleshooting already. We'd like to see how we can assist.
To double-check, are there any non-iPhone users in these threads?
Also, when you see they're breaking up into separate texts, is it separating into different groups, or with individual users?
Under Settings > Messages, make sure that both MMS Messaging and Group Messaging are turned on. Let us know if this helps, and the details above, and we'll continue from there.
Best wishes.