An update, just in case this would ring a bell to someone.
First, this jumpy playhead symptom, when it appears, seems to mostly affect the second half of 30+mn clips.
Second, sometimes, the offset between the point clicked and where the playhead stops is nearly consistent : 20 seconds and 2 or 3 frames.
Looks weird enough. I converted the whole clip where needed, but I’m still a bit puzzled as to why this happens, on about 1 out fo 4 clips. Again, these are footage from SONY CX-305 and MC1P cameras.
FWIW.