When recording an external source inside an audio track (selecting the Ext source in the track routing), there is no input monitoring available, if ASIO direct monitoring is switched on.
When the track input button is switched on, there is no audio signal, and no input metering available.
But the audio is recorded.
This mean that Nuendo does not transmit the ASIO direct monitoring information to the audio interface for external inputs.
Something that prove that this is a bug and not normal behavior is that the channel input is muted. This is the normal behavior when ASIO direct input is active on a Nuendo channel.
So this mean that only half of the ASIO function is working for external inputs : channel input is muted, but the interface is not switched to direct monitoring and does not send its level metering to the Nuendo channel.
When disabling ASIO direct monitoring, audio input and metering is available again.
Last, if the same physical inputs are routed to a Nuendo input, and this input selected as the track routing source, then ASIO direct monitoring works normally.
This show that there is a problem with ASIO direct monitoring for external inputs. Probably the ASIO direct monitoring switching code is simply missing for external inputs.