Hi,
I have much older iChat Saved Chats that have idents like that.
It looks like the info should appear with the Time Stamp or at least centralised like the Time Stamp as it obviously does not appear always at the same time.
Try giving the chat a Name.
In the open chat use the Details option top right.
You can name the Chat here.
Example
This may stop the App from trying to inform you it is a group chat.
It might also do this if that particular person goes Off line.
7:43 pm Monday; October 1, 2018
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
MacBookPro 15" 2016 (High Sierra 10.13.x)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)