I draw the following two shapes in .9.02.
I select the thin slanted piece first, then select the long square/rectangular-ish shape. If I immediately press the Boolean Subtract button, I get this:
I then ctrl+z to undo. I then try Weld All Shapes Together. Undo that. Then try Boolean Union. Undo that. Then retry Boolean Subtract. I thought I would get the same result as when I did Boolean Subtract the first time because I have always done an undo before trying a new operation (the selection always stayed active, so I never had to redo/change that). Instead, I get this:
You have to do this in the steps I listed to get the different behavior. There may be other sequences that produce the same or other behaviof but I haven’t had time to test.