UPDATE:
Possible workarounds but this is certainly a bug as reported by ‘Deathrey’ on the UnrealSlackers discord server:
“There is a bug with static mesh data interface and audio spectrum data interface, making former think that it is not what it is, causing a bit of incompatibility. Suggest you fill out bug report form with the repro project attached. From workarounds this far, you should be able to 1) Use skeletal mesh instead 2)Have two emitters, one, that has particle positions derived from static mesh, and the other one, which would spawn using positions of 1st emitter using attribute reader and will do movement logic according to audio spectrum. 3) Bake audio spectrum every frame to either particles/grid 2d/array data interface or even a standalone render target. Then sample baked spectrum for this frame instead of using audio data interface in emitter or system, where you need static mesh data interface. My choice would be the latter.”