Have you tried clearing the cache on the message app?
Have you tried resetting your phone? ( Backup your data first)
could you share your device's model number, software version and the carrier you are with?
i haven't had to send texts to 10+ people at once in a group message, but I want to take a guess and say it's something the carrier can help. Just an assumption but I feel like the carrier could limit the group messaging to lighten the load on their server or something, so check if it's allowed in the first place... if they say it should work, then what you can do is submit an error report using Samsung Members after opening up the messages/chat again to try replying... providing screen recording or snapshots would certainly help.