There doesnt seem to be some Glaring Errors in that Particular Object, or the Biome Config.
I have two possible things:
I observerd some Placement Issues on my SkyLands with: "SpawnHeight: highestSolidBlock"
They didnt appear to Spawn at all, I didn´t really look into it on what was Causing it, as I went just for "RandomY"
But I would think that Highest Solid Block doesnt work with SkyLands.
The Screenshot where the Objects get Generated Correctly the Blocks get down to 0, Where they dont Generate, I cant see it on the Screenshot, so that Could be the Issue.
It could also be a Biome Error, is there maybe another Biome? Doesnt Look like Chunks getting not generated. Looks more like a BiomeBorder.
(I just assume you remove the Region Files each time you make Changes)
Apart From that, I have a Suggestion regarding your RessourceQueue.
While: "CustomObject(UseBiome)" is easy to use, its not that Flexible.
Especially if you want to adjust the Overall Terrain.
You can use More CustomObject() ressources per Biome. I would suggest moving it to the Top of the RessourceQueue and add the Objects you actually want to spawn.
That way you can also Increase or Decrease the Amount of Objects that Spawn.
And Also you have a more fine Control over the Order in Which the Objects Spawn.
After another Look. It really looks like you didnt regenerate the World. And the Chunks were Previously Generated with other Settings.
The: "SurfacePatch(MOSSY_COBBLESTONE,AIR,198,220,STONE)" doesnt cover the whole Border either on the First Screenshot.