I've had this issue since upgrading to Big Sur. I'm in the same boat as @M_Prewitt... tried sharing an item with contact... works temporarily need to try doing a tap-back reaction... that might be the quickest solution. I've been starting a new message and then adding my wife's name, then starting a new message that way... the reply box appears... temporarily. The crazy thing is that I only have this issue with one person. One contact. My wife... which happens to be the person I message multiple times, all day long! that's the most frustrating part!