I've had a bunch of these too (sent in a couple logs). Scenario - create one Thought, then creating a child thought... as entering the child though, in the background the plex re-draws and the first Thought disappears (poof!). If you continue with the child thought, it crashes. If you notice and escape out, it does not crash but the other thought is deleted. Sometimes I think a pre-existing thought gets deleted too -- I notice this because I am left with a lot of Orphans.
I just switched off of Alpha, so maybe won't have so many crashes.
I am getting back into the Brain after a long hiatus. Looking forward to things to come with v9.