Asteraoth, thank you for your speedy reply. I tried searching before reporting, but couldn't figure out how to format my search query to bring up any results and would have had to look through the entire registry of reports manually to find if the issue had been previously reported.
If encouraging people to search before reporting, maybe it would be better if they could do a simple search instead of having to format it. There are a lot of kids and people with little to no technical or programming knowledge that play this game who would still like to be helpful when they find an issue.
The lava had burned off the trees in the area before I found it.
After re-creating the world with fire tick turned off, it seems the stone formed due to overhanging branches and leaves. I'm not sure if that is as intended.