Group iMessage sender shows up as Unknown, but all sender/recipients are actually in the Contacts list
On an iPhone 7 Plus, at iOS 12.1, I deleted some group messages that were from a sender (J) to myself (M) and 2 others (A, G). I then got another group message from J to M, A, and G, but now the sender is indicated as Unknown. Contacts J, A and G are still in my Contacts. How do I get back to their names being displayed? I did the turn off/on "Send iMessages" setting, but to no avail. Anything else to try? Thanks.