When things appear/disappear with no apparent cause, I always wonder about iCloud.
Getting these things from a Time Machine backup looks difficult. I guess that's why Apple punted and made this kludge:
It looks like the Reminder data are stored somehwere inside ~/Library/Calendars in a folder that ends in .caldav.
You could try going there, then entering Time Machine and restoring it, but I don't know what else will get rolled back besides the Reminders.