I understand that not all mobile devices are supported. in terms of working out whether it is wpt or the device that is at fault this message seems to indicate that it’s the device in terms of diag:
adb STDOUT: Encoder stopping; recorded 0 frames in N seconds
nothing else other than no frames were recorded, so the filmstrip is empty. this doesn’t affect whether the screencap png works or not.
either in the wpt agent or by observing it in the device shell directly when a screenrecord finishes.