When I respond to group messages from the notification bar, my group response is received individually. After searching the Samsung Community, this seems to have been a recurring defect for years, with many quite involved solutions.
It should just work. If I go to the main screen of my messaging app, the one that came with the phone, click on the three dots, click settings, click more settings, click multimedia messages, and toggle the "group conversation" on, IT STILL DOESN'T WORK. Even if I toggle "group conversation on and off, reboot....etc. it still doesn't work.
However, when I first receive the message, if rather than immediately responding from the notification bar, I go up to the group listing at the top of the screen, hit the three buttons THERE, and toggle the "Group conversation" button (which has been on), off and then back on, IT SENDS TO THE ENTIRE GROUP. I guess I will have to do that EACH time for each group message.
I have been a Samsung user for many years, and really like most of my phone's features, but have regularly noticed button switches losing functionally. Ambient light adjustment for screen brightness.....lock screen avoidance from secure locations.....I have to regularly toggle these switches off, then back on to get them to work again.