Sample world: wrld_aa458f70-972b-494b-9eaf-6f6b1483ca9b
As demonstrated in attached videos, relative elevation sitting at around 45° from horizon causes spatialised non-HRTF sources to hard-pan to dominant direction. This is most likely an issue with the middleware itself.
In theory, these deadzones would be eliminated using post-effects such as Reflections, Reverb mixin, and Transmission, but direct audio should not rapidly change channels like that.