Can't figure out what's going on here, but my Macbook Pro (mid-2011, macOS Sierra) no longer remembers anything new in Messages. Every time I quit and relaunch Messages, the history is right back where it was before I launched. The last messages I have there are from Feb 4 (4 days ago) and anything since then is always gone. I can open it right now, chat with someone, then quit & relaunch and everything we had said since launching the app, is gone. I did just do an OS reinstall recently to try and fix some other random weird issues I was having, also repaired all permissions etc. though there was nothing to repair. I'm stumped.
EDIT: Additionally, now every time I launch Messages, my own chat (with my contact) is defaulting to an alternate email address of mine, which is not iMessages enabled, so appears in red. I can change it to my valid address, send messages do whatever I want... but again once I relaunch, it's reset to as it was before, with all new chats gone and seto to use a non-enabled email address.