Your notching the Chip Board? And trying to line it up with the wire laying OVER the ‘beam’. If you’ve got to, notch the ‘beam’ instead. Really, you aren’t nailing the chip board down, it can move. A half inch of notch in a stringer is nothing.
Hmm… Could the guest bed room be an addition?
The ‘beam’ the wire is going over, how big is it? Is it the stringer on a pre-built truss? Is it a 2x4?
There is room under the ‘beam’ that you could pass the wire through? So drywall is not nailed to the beam from underneath? Is it a dropped ceiling?
The wire, should have some printing on it. 12-2? 14-2?
If your going to cut wood, the chisel and hammer is probably the way I would handle it.
The best way would be to reroute it. But you say it can only move 2cm side to side. That’s a bit tight. I would try to follow it and see where it goes. You may be able to unhook it from an outlet, reroute it under the ‘beam’ and have plenty of wire with out having to use a junction box.