As a suggestion... How about turning magma into obsidian via bucket-brigade? I heard somebody say that it didn't work, but I'm not sure how full the magma tile was... I think magma only turns into obsidian once it reaches 1 or 2/7 before coming into contact with water, with any water added before that turning the water into steam and making some magma disappear. Not positive though. That might be worth looking into as well.
I've tried many times without success. 7/7 magma burns off the water, 1/7 does as well. I've even created 20 buckets to see if I could get several dwarfs to dump water on it at once figuring it had to be 2 units of water. None of my experiments have worked with this method so far.
Bucked brigade works qite well with obsidian making. You just need to know a little trick: When you designate the pond on the tile above magma, it does not work: dwarves insert water from buckets just inside magma, some check doesn't work -> no obsidian. However, if you designate the pond one level above that, water from buckets will be thrown at the tile ABOVE magma, then fall into magma via gravity. The check works -> you have obsidian.
There's a fort under my sig, it has a POI "Project Octahedron". The obsidian columns in the lower part are made this way.
And one more thing: when you have only 1/7 of magma it will disappear without creating obsidian.
So this bug only really kicks in when you build a plot completely underground at first, then channel out only part of the roof and cover it again without remaking the plot?
It's not a bug. Part of your plot is aboveground and another part is belowground. You can plant aboveground crops on the first part and belowground ones on second part. Absolutely logical.