Recently I had issuse sending and receiving text messages in a group. The messages from my end would send to each individual instead of all together and I would receive messages individually aswell. Recently I found that apps like Textra and GoSMS Pro can fix this issue. On Textra go to the settings and under MMS check the box for "Enable MMS for Group Chats" and on Go SMS I believe there is such setting option as well. If you like using the stock app you can still install either app and turn off notification on the apps to continue using the stock app and the group message will keep working. Hope this helps