Hello,
Using Zenphoto 1.5.9.
If you restore a database using the Zenphoto admin page, you loose your last tag. You have to recreate it.
Luckily, images and albums using this tag do not loose it.
To bypass this, I've created a tag named z-zzz. (last alphabetically for me)
Restoring database from my online website to my localhost machine = no more z-zzz tag. But as I do not need it, I do not care.
If you restore your database on the same machine (backup -> restore), then you have to think about it !
Well, itt all depends when your database backup was performed. If you add things after the last and before the next that naturally gets lost.
The included backup tool usually performs an update once a day (if the site has been visited by anyone front or backend). If you lots of change iI would recommend to manually create backups afterwards.
This is how to get the problem :
1 add a tag named zsomething (last tag alphabetically in your list)
2 perform a backup - it contains your tag.
3 restore your backup.
You will loose the tag you added in step one.
Note: you will loose your last tag, even if you added it 3 weeks ago, and performed your backup very recently.
It just needs to be the last one in alphabetic order.
I'm using 40 tags. My unused z-zzz tag was added 1 week ago, because of loosing my previous very old and needed tag named z-monde.
I also supposed that it was due to tags beginning with z- , but did not check !
I can see my z-zzz tag if I edit the uncompressed backup file :
Once restored, it's no more present :

Did you try on your side ?
Note the z-zzz tag is just present on my online website because I know it will disappear on my localhost machine.
Before using this z-zzztag, z-monde disappeared on my localhost machine once the database was restored.
Joyeuses Pâques - Happy Easter - Frohe Ostern
Yes,
I always add content on my online web server first.
I use Clonezilla to update my localhost machines (2) (my backups)
Then, I restore the 2 localhost databases.
Second backup is on a laptop I use to show what I did (no need for dsl or fiber connecction.)