Group chat reply box disappears on only one group iMessage chat

For almost a year, I have one (only) iMessage group chat in which the reply box disappears. All with iPhones. I can start a new message, and after a time (varies from one hour to a day), the reply box disappears. Apple Help has not been able to figure it out. It happens on BOTH my iPhone and my MacBook (although at different intervals). Help!