QAudioInput sometimes not writing to QIODevice

I have a problem with QAudioInput sometimes not writing to QIODevice. Might be an accident, but after trimming it down it turned out that the problem is 100% gone once I got rid of ui...

Attached example code is heavily trimmed down examples/multimedia/audioinput. The version basic_audio_test-no_ui does not have any ui. The version basic_audio_test-with_ui opens a main window with empty combo, but then never uses any elements of the ui and accesses the QAudioInput in exactly the same way as the no_ui version.

The example code in both versions basically starts QAudioInput for the first device found and for QIODevice defined in soundbuffer.h/cpp, which in writeData() only writes out a message. The problem is that in both cases it seems that QAudioInput is opened and started without errors, but then in the case with ui I (sometimes) do not get the debug from writeData() (so I'm almost certain this method is not reached - that's why I started investigating/trimming my case down in the first place => in a more complex application I was not getting any data).

• in "no_ui" version, in 100% of cases I get a stream of QIODevice's writeData calls:
  basic_audio_test-no_ui> ./build/basic_audio_test
  buffer opened
  "slotDeviceChanged(0:front:CARD=NVidia,DEV=0) reached"
  DFLT not supported
  About to START audio input
  Audio input started
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  ^C

• but in "with_ui" one of three cases happen:
  1) I sometimes get the same:
  basic_audio_test-with_ui> ./build/basic_audio_test
  buffer opened
  "slotDeviceChanged(0:front:CARD=NVidia,DEV=0) reached"
  DFLT not supported
  About to START audio input
  Audio input started
  "SoundBuffer::writeData() : max_size == 2560"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"
  "SoundBuffer::writeData() : max_size == 1280"
  "SoundBuffer::writeData() : max_size == 1920"

2) BUT sometimes I get just:
basic_audio_test-with_ui> ./build/basic_audio_test
buffer opened
"slotDeviceChanged(0:front:CARD=NVidia,DEV=0) reached"
DFLT not supported
About to START audio input
Audio input started

and then no more output (ui is responsive, btw).

3) and sometimes I get additionally 1 line with writeData and nothing more:
basic_audio_test-with_ui> ./build/basic_audio_test
buffer opened
"slotDeviceChanged(0:front:CARD=NVidia,DEV=0) reached"
DFLT not supported
About to START audio input
Audio input started
"SoundBuffer::writeData() : max_size == 1280"

and then no more output (ui is responsive, btw). It never happens that e.g. 2 or 3 or other number of frames is received. It's always - either unblocked stream, or nothing or (rarely) one frame.

As I wrote - I have noticed the problem (no data received by IODevice) in a more complex setup which does not depend on usage of terminal output (so it is not just about writing out messages using qDebug()...).

The first question I asked was - is there something blocked or is there something looping or...?
I attach three strace outputs:

• from "no_ui" version (which never stops writing messages about new data received);
• from "with_ui" version in the case when it does not stop writing messages about new data received (seems quite similar to the above;
• from "with_ui" version in the case when it does NOT write messages about new data received (it definitely does "something" - hard for me to say what...)