A Polarization Dependent Loss or ‘PDL’ issue usually indicate that dust has made its way into the OSA, say, onto the mirrors of the monochrometer and polarizer.
One indication of dust internally would be seeing a lot of dust on the optical input connector – this is the first clue.
Repair involves a full disassembly and cleaning to return them back to “as new” performance conditions.
(this usually takes several hours; so it is not minimum (two hour) repair.)