Hi,
It is the correct behavior, it is not possible, because it is not yet implemented. The custom field filters for now only work with CPT and User type of results. Taxonomy meta only had been recently added to wordpress – it was previously only bypass supported by ACF.
Sooner or later this is going to be implemented for taxonomy term results as well, I am working on new features as fast as possible. Currently there is no way to do that via this plugin.