I just took a flight CGK-KUL-AMS-BRS on a light fare. The CGK-KUL leg was KL coded (KL4115), but operated by MH. When I booked the ticket, 1 free bag was indicated (instead of 2 for FB plat) and there was no problem checking the bag in at the MH counter.

I have another flight BRS-AMS-SIN-KUL with a light fare, with the SIN-KUL leg on SQ (and not a AFKL codeshare). My ticket again restricts me to 1 free bag - there is a note saying this is due to the partner flight if you try to buy an additional bag. The same 1 free bag applies if I opted for a SIN-KUL leg with MH (not AFKL coded).

For the return, as far as I can tell, any MH flight (no codeshare) has 1 free bag. I get two if I take the direct KUL-AMS-BRS.

Hence, it's probably best to always check your allowance when you buy the ticket, although I have not encountered a scenario where a free luggage wasn't included in a light fare with FB status. Yet.