I have the same problem on my Note 3. I have been sending large group text messages (15 - 30 recipients) weekly for months (using Textra) until 3 weeks ago. Suddenly they started failing, would not send. It took me 2 weeks to figure out that 10 recipients was my new max. I didn't change anything(carrier, phone, app), group texts of 11 or more just fail now. I've tried 4 different apps, used Wanam Xposed on my rooted phone to remove the 10 person limit, and nothing works. It's all pointing to a carrier limitation (Straight Talk on ATT). But I don't know what our if they changed something.
This is ridiculous, basic function that I suddenly don't have.