V6 - Boolean Split

In one file I have successfully split a solid. I then go to an earlier version of the solid and try the split. Everything appears to go fine as there is no fail message. But when I try to select only one half, I can only obtain both halves. It is as if they did not split.

If I try to repeat the split, I can now select only one half, but the process then the split process fails.

Any idea of what is going on?

without the file is quite hard to see the problem…