Is the "initial-zoomout-validate" config value supposed to be working? i did a quick search in the repo, and DynmapWorld.doInitialScan() does not seems to be implemented anywere.

If not, do you have any workaround to rebuild the zoomout tiles without doing a fullrender again? i'm using a mysql db.
I was wondering if i remove the zoomed out tiles there would be a way to rebuild them.

Manually purging the files should work, but you then have to rerender those tiles again

Any updates on this?

I fixed this issue by following the steps unde this reddit post

Modify configuration.txt in this way:

#Filetree storage (standard tree of image files for maps)
type: filetree
#SQLite db for map storage (uses dbfile as storage location)
type: sqlite
dbfile: dynmap.db

Add a plugin

I have also added this plugin. I don't know if it is necessary but it worked.

Start a fullrender

Self explanatory

That plugin is the main thing that's different and potentially helping in this case. This issue otherwise happens on both filetree and sqlite storage options (although from personal experience seems to happen more often for sqlite).

I'm interested to know what it does different/how it helps.

EDIT: as per the description from the dynmap readme this is not really a new thing or a fix.

†Note: drivers for SQL are usually included for Spigot and its derivatives but not included with other platforms or Dynmap. For Forge and Fabric servers we recommend Kosma's SQLite mod or MySQL mod to add the needed drivers. Additionally, injecting driver classes into jar file will be recognized and supported.

So if you are using plugin-based servers you are already good, as this should only be needed for fabric/forge. Moreover I would imagine that in those cases it would flat out not work at all if this was missing, but I don't know for sure.

I fixed this issue by following the steps

In your case it was most likely just pure luck.