I can confirm that.
There is a command, which HTC uses to activate the camcorder mic. Maybe HTC forgot to call this procedure when restarting the 2nd recording.
The function is already present in the phone application to enable the camcorder mic for the noise canceling function.
I use this as a workarround to enable stereo recording.
Maybe someone can compare the old and the new video app to find the "enableCamcorderMic" Function and make it available for other applications. There are some applications which allow to select between the main and the camcorder mic. They work on most devices, which have 2 mics, but not on the HTC ONE. This can be tricked out by switching to the phone application while a recording-application already records. Start a call and cancel the call. Then return to the recording app via taskmanager and you will have 2 channel recording. Unfortunately this does not work for the video app, which cancels video recording when switching to the phone app. But I did not need the workarround in the past for the videoapplicatiom. But now HTC has a bug in it. I hope someone could be able to isolate the "enableCamcorderMic" function.