I understand I /should have/ used a different app for backing up my messages.
I did not. I used Titanium, and made full backup of apps and data.
I was apparently also using Android Messenger and not Google Hangouts to store my SMS. This service was apparently deprecated in August.
I go to Titanium to restore my messages, and for a moment, it looks good! I see all my messages. Then they rapidly delete themselves before my very eyes!
I try again, I reload app+data, or data only, and the same thing happens---I see all my messages, then I watch it delete them one by one very quickly.
What is happening in this process, and how can I prevent it? Where are they going?
About adb backups, not TitaniumBackups, but is this relevant (could I get my SMS back if I have an old ADB lying around?)? https://www.reddit.com/r/Android/comments/3nrork/adb_backup_and_restore_not_all_its_cracked_up_to/