This seems to have happened between 5.2.1 and 5.3.1, but it's still there in 5.4.0.
I want to use tree support with brims on the support, but not on the model.
It worked in 5.2.1, even though it only added the brim internal to the tree.
In 5.3.1 and 5.4.0, "Enable Support Brim" with "Build Plate Adhesion Type" == "None" will result in a tree support without a brim, and it hoovers 1 layer above the build plate. And will of course not stick to the plate.
Workaround: Use "Build Plate Adhesion Type" == "Skirt", and the internal brim in the tree is back.