From the spec:

power_level_content_override: The power level content to override in the default power level event. This object is applied on top of the generated m.room.power_levels event content prior to it being sent to the room. Defaults to overriding nothing.

It's unclear what the exact override semantics are here. Is the entire generated m.room.power_levels content discarded and replaced with the object specified here (ie. potentially removing keys)? Are they deep-merged? If yes, what are the exact merge semantics?