To Moderators : I suggest we move this discussion to the lounge thread.

To OP : Welcome to Flyertalk .
This is indeed weird and totally abnormal IMO. Usually (speaking in general), it's the other way round : you have access when flying your airline metal but not when flying the code-share.
MLA has always been a complicated case, you are not the 1st person reporting problems over there, whatever you are on AF metal or on the code-share. It seems really weird that there is an agreement between KM and AF for lounge access with the code-share but not with the AF flight. Could it be a capacity issue of the KM lounge ? But it's not that AF is flying multiple times a day to MLA. Just one seasonal flight, not even sure it's daily.
You should definitely write to AF about this.
Regarding your other comment about service decline for the last 6 weeks, that's certainly not my experience, but without more details it's hard to elaborate.