I suppose it was changed and undocumented with 04/08 patch.
Now, you can only save the progress on ONE SINGLE MOD MAP per save slot.
Before, you could save as many as you wanted on a single save slot. And that was good.
We only needed a way to easily manage each map inside the same slot (to easily reset one for example)
I play 10 different maps, one day ill certainly play 30. Will i need 30 different save slots? Given we currently only have 4, and 3 of them are used for different playthroughs... this looks completely bad, unmanageable, unfriendly.
So, please, bring back the old system as soon as possible.