I've just had the same issue and tracked it down to a line of code in akWwiseXMLBuilder.cs
In that file, there was a line at the start of the function SerialiseSoundBank that read:
var includedEvents = node.SelectNodes("IncludedEvents");
However, my SoundBanks.XML didn't contain any nodes with this name.
I changed this line to read the following, and I am now seeing the attenuation spheres.
var includedEvents = node.SelectNodes("Events");