I hate to be one of those who answers his own question, but I’m sure others on here are also looking for the answer, given the number of views.
What I found is that if neither AffectsHeightmap or AffectsWeightmap are true in the class defaults (accessible in class settings) then the brush won’t spawn like that.
Simply making one (or both) of those true in the class defaults makes it so you can spawn the brush just like the landmass ones.