For anyone else experiencing this issue, it does appear to be a bug.
onNewManufacturerSpecificData is called unless requestBasicMeasurement or requestAdvancedMeasurement in AntPlusWightScalePcc have been called. Once the measurement has been completed (scale sends at least one 0xFFFF weight), onNewManufacturerSpecificData will resume being called. (This appears to be because when a measurement is requested the ANT+ Plugins Service internally starts using ChannelTask_BasicMeasurement or ChannelTask_AdvancedMeasurement to process received data, and they don't handle manufacturer specific data pages).
As you can imagine this behaviour is rather useless, as the manufacturer specific data for this weight scale is sent only during a measurement. From taking a quick look at the other plugins I think this might only affect weight scales, as most of the other plugins don't appear to use custom AntChannelTask objects to the same extent the weight scale plugin does.
Unfortunately I can't see a way to work around it until it's fixed in the ANT+ Plugins Service.