Any reason Obsidian Plains are missing from the old verison?

[Uberness]

In a recent game I summoned the infernals, they came next to a large desert and floodplain area, and turned all the floodplains in the middle of the donut map into normal desert, there used to be a special hell terrain for floodplain>hell floodplain, where did it go?

 

[Blackmantle]

They seem to burn away when they turn burning Sands (which like the name implies do burn. flames kill off other improvements and thus sadly obsidian plains as well it seems.)

It seems a bug, one way or another...

Try vitalizing those floodplains to plains + floodplains and have a look if the obsidian plains stay then. Maybe...