It is not burned bulb, I checked and I had it few times before
As I said and Chris Scott also obviously knows about this, it's something with FWC misbehaviour but I don't remember exactly what and there is no reference in FCOM. I think that it's in AMM but I don't have it
If I remember corectly, FWC1 is wired to supply Master Warning and Master Caution upper bulbs on CM1 side and lower on CM2 side. FWC2 does the opposite.
I would appreciate a reference