I'm actually a native English speaker (30yo), and feel this might actually be a recent usage? I think I started noting it as an oddity in the last decade or so. I kind of like it for the reason mtlynch said, even though you could argue it's less correct than "can't recall". I'm curious if there has actually been a long usage of "forget" this way that I managed to be oblivious of, or if it actually is recent.