Connect to the device via ADB, they logcat the output and look for what keyevents are being triggered.
If the kernel "consumes" the keyevent, it will not be forwarded to the foreground app.
Logcat able to see some of system UI events related volume bar.
2019-08-05 14:24:19.786 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=0 level=4 oldLevel=4
2019-08-05 14:24:19.786 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=3 level=13 oldLevel=12
2019-08-05 14:24:19.787 4658-4775/? I/chromium: [4658:4775:INFO:media_manager_impl.cc(464)] New volume for MEDIA: 0.866667
2019-08-05 14:24:19.788 4658-4658/? I/chromium: [4658:4658:INFO:metrics_recorder.cc(307)] Count histogram: SetVolumeLevel.E8C28D3C, sample=86, boundary=101
2019-08-05 14:24:19.792 3441-3441/? I/vol.Events: writeEvent show_dialog volume_changed keyguard=false
2019-08-05 14:24:20.591 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=10 level=14 oldLevel=13
2019-08-05 14:24:20.600 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=9 level=14 oldLevel=13
2019-08-05 14:24:20.600 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=9 level=14 oldLevel=13
2019-08-05 14:24:20.600 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=8 level=14 oldLevel=13
2019-08-05 14:24:20.601 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=7 level=7 oldLevel=6
2019-08-05 14:24:20.601 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=6 level=14 oldLevel=13
2019-08-05 14:24:20.601 3441-3631/? D/vol.VolumeDialogControl: VOLUME_CHANGED_ACTION up
2019-08-05 14:24:20.606 3441-3631/? I/vol.Events: writeEvent level_changed STREAM_BLUETOOTH_SCO 14
2019-08-05 14:24:20.606 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=5 level=7 oldLevel=6
2019-08-05 14:24:20.606 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=0 level=5 oldLevel=4
2019-08-05 14:24:20.606 3441-3631/? I/vol.Events: writeEvent level_changed STREAM_VOICE_CALL 5
2019-08-05 14:24:20.606 3441-3631/? D/vol.VolumeDialogControl: onReceive VOLUME_CHANGED_ACTION stream=3 level=14 oldLevel=13
2019-08-05 14:24:20.606 3441-3631/? I/vol.Events: writeEvent level_changed STREAM_MUSIC 14
2019-08-05 14:24:20.607 4658-4791/? I/chromium: [4658:4791:INFO:volume_control_android.cc(218)] SetVolumeOnThread: level=1 (1)
2019-08-05 14:24:20.607 4658-4791/? I/chromium: [4658:4791:INFO:audio_sink_manager.cc(77)] SetTypeVolumeDb: level_db=1 type=kOther
2019-08-05 14:24:20.613 4658-4791/? I/chromium: [4658:4791:INFO:volume_control_android.cc(218)] SetVolumeOnThread: level=0.933333 (1)
2019-08-05 14:24:20.613 4658-4791/? I/chromium: [4658:4791:INFO:audio_sink_manager.cc(77)] SetTypeVolumeDb: level_db=1 type=kMedia
2019-08-05 14:24:20.616 3441-3441/? I/vol.Events: writeEvent show_dialog volume_changed keyguard=false