What I’m seeing, though, are forms like “I was read this book” or “These short story are….”; they’re in papers written by native English-speakers who don’t speak any of the dialects that would normally drop those suffixes, and the same students do use the suffixes in speech (it’s exactly the reverse of the usual situation, in which students don’t know they have to write bits that they don’t say). If I draw attention to a line in which there’s a missing –ing, etc., the students frequently *can’t* see anything unusual about it;

I'm afraid I can't see anything unusual about your first example either. What am I missing?