In the ten+ years I have played with WoW.. this has only happened about a hand full of times.
The saved data file for an addon gets reset to no data.
I have been doing census runs on Realm/Factions that have very low census run counts (zero).
I had been at it a few hours when I started thinking about needing to upload, followed by oh I missed the process time.. I will add a few more runs before I upload... [bad mistake]
I discovered something was wrong when I moved to another new realm... and CensusPlus didn't start the census run.
Checked the options only to find them at default never run before settings.
After the census finished and I logged out, I checked the data file.
Yup.. only the one run.. all settings, history and all gone or reset as I had modified to do the one census run.
Sigh over a dozen census runs lost forever.
Like I said this is something very rare.. and it isn't addon specific.. some of the lost addon data hits were from before I ever knew about CensusPlus.
But it is a side effect of a poor backup choice by Blizzard.
Most people who save data and keep backups will do the follow.
Does backup exist.. Yes .. delete
rename current data as backup
save new current data.
Blizzard does this
Does backup exist. No rename current data as backup
save new current data.
So unless you are deleting your backup files frequently, the backup you have for the addon is probably so old it is worthless for your needs.