I'm seeing problems again stepping back through the Undo graph. You can 'ctrl z' once, and when you step back again, you start losing positions at the front end of the graph. If you click to a past position say 4 positions back, you will lose multiple positions at the front of the graph. I thought this might be specific to the 20210513-dcafd2c0c1-win_x86-devprev build, but I also see it in the last main build too.
It seems to be specific to having the graph length set to the minimum 25 positions, in both builds. If, say, you forget you set the length at 25, run your magic, and then change the graph length to say 2, 3, 4 hundred, and then start stepping backwards, chances are you will crash. If you run your magic at length 400, then reset the length to 25 and start stepping backwards, you will see the positions disappearing again, or you might crash.
Take home message, dont set graph length to the minimum 25 in early game. Only reset later when you need to run 'processing scripts' where you need to just let it do its thing