Wwise produces ambisonics signals when one of the busses' Bus Configuration is set to Ambisonics. These signals are decoded, meaning that they are transformed into a standard multichannel representation, such as stereo or 5.1, whenever a parent bus of an ambisonic bus has its Bus Configuration set to such a standard configuration.
The Master Audio Bus inherits the configuration of the endpoint, which depends on the platform and is usually detected automatically. Most platforms only accept a small selection of output configurations. These are typically stereo, 5.1, and 7.1. So, in general, the Master Audio Bus has a standard channel configuration, which forces decoding of child ambisonic busses to this configuration. This means that in most cases Wwise will not output an ambisonics signal by default.
However, it is possible to write a custom Audio Device plug-in that declares an ambisonic configuration. In this case, the Master Audio Bus would have an ambisonics Bus Configuration and Wwise would preserve, not decode, the ambisonics signal.
Certain endpoints accept ambisonics signals. In these cases, the Master Audio Bus can inherit an ambisonics configuration. Refer to the platform-specific sections in the Wwise SDK documentation for details.
Alternatively, if you only want Wwise to save the ambisonics signal to a file, you can simply insert a Recorder on an ambisonic bus. The generated file will be in FuMa or AmbiX format, with ambisonic order equal to that of the bus hosting the Recorder.