My exact same problem started with Sonoma and iOS 16. Now I am on Sequoia and iOS 18.3 and persists. I have reported it to Apple and saw the Geniuses at the SF Union Sq flagship store - they were completely puzzled with no ideas other than the many I already tried. Only temporary solution is to delete the group, along with the thread of messages and pictures, recreate the group and reply. Lasting only for between 20 minutes or an hour or two. Frustrating!