This strange. I have completed the game twice now and got all achievements. But in the first game I played there was an additional menu next to "Select Save", called "Select Level", I tested this and it let you select maps, I assume the DLC maps.

But I no longer can see that menu entry. So I cannot access the DLC... and yes the DLC *was* and *is* installed.

This might have to do with my 1st game run, when you select that 100% save, and quit the game, only to relaunch it, you end up in the scene where you find the row of retracting cryopods. Hmmm...

When you relaunch the game instead of ending up in the main menu, with a "Continue" menu entry, you are forced to continue the game at the last point you left off. Thus you cannot access the "Select Level". Seems this happens once you completed the game. And it happens for both save slots for both completed games.