I have checked this today and my findings are:
a) the spawner volume bottom must be placed exactly below the surface where you plan for the spawn to occur
b) supposing your landscape there are places above the most bottom part where you plan to have trees, then you need to adjust the scale of your volume to be lower of that if you plan to not have trees or higher than that if you also want to have trees there.
Usually env artists set a different spawner volume for different altitues because usually the flora differs
The error you described in the first post occurs with my finding at “a”, but supposing you already knew that, then everything points to your own discovery of the Procedural Foliage Volume not working as intended for the world composition made piece by piece, and as it is just still an experimental feature, you might want to report it as a bug and also explicit the steps of reproducing it, there is a new form for bug report atm.