However the selected altitude that some ATC systems can see through the Mode S ES is referenced to the QNH set on the FCU even if the altimeter is in STD mode.
Is that the bug reported about 5 years ago?
I understood so far 2 values are transmitted. The FCU selected ALT and the baro-pressure setting. For some weird technical reason, the S mode transmits the last QNH at all times. ATC systems at the moment ignore the selected pressure value, so no harm is being done yet.
Not sure where to find the original Information Telex from Airbus, sure someone has access to it ...?
note: mode C transmits the STD-referenced readout of the ASI at all times.