This can happen when the audio-jack isn’t completely plugged in, theres a small switch-contact inside the plug-hole which did the job of mechanical/electrical re-routing of the audio-out-signal.
Have you confirmed it with different headphones? What sailfish app are you using to play audio, does it happen on different apps? I tried mine C2 and it properly routes audio to headphone/speaker.
Today during a call I noticed that too. Headphones are from former Xperia. But I doubt that this is relevant.
Will try to observe each time and report.