We also experienced this on the current generation of our own forest, which I wiped and restarted about a week ago (the previous server generated them just fine). Our server community searched every inch of our forest and only found one birch tree, confirming the spawners exist, but they were either overridden by other spawns during generation or are rendered inaccessible.
As a workaround, I used Prefabulator to drop some into our server using the Tree Spawner - Any Birch prefab, set them to ground, then reloaded the chunk in my client. Not a fantastic solution, but usable for now.
I also noticed that no save wipeforest appears to exist when I searched for it. Given the alternative is to wipe the entire server, is this command expected in the future?
Edited to add: our forest in my instance has plenty of “static” birch trees, but no spawnables that we could find other than the one.