How to get Arcade map?

Hi!

I would like to complete a level 12 Arcade map.




Currently I do not have any Arcade12 in my stash. (I am on SSF.)

In order to drop Arcade12, I have to complete a neighbour map. Either level 11 Necropolis:



or level 12 Malformation:



At least I believed this until now.


In my map stash, there is no Necropolis11.



There exists Mausoleum11,



there exists Silo11,



but there is no Necropolis11. Not simply "zero Necropolis11 yet". There is no even zero Necropolis11. Not even place for that map.

Similarly, there is no Malformation12 map in my map stash.



There exists Leyline12,



there exists Museum12,



but "Malformation12" map does not exist. There is not even place for it.

So what is going on? I would like to understand.