I am studying cognitive linguistics and in there was a lecture where lecturer said one interesting thing. There are two sentence:

1. This is the report that I filled before reading.
2. I filed the report before reading.

They are both pretty understandable and look pretty normal for me, but lecturer said that any native speaker would say that the second one is ungrammatical and wrong, but it is unclear why so.

Is it really ungrammatical and can someone try to explain why is it felt so?

The second one would sound weird to me. "I filed the report before reading it." or "I filed the report before I read it." both sound much more natural.

I think it's because in the first one, "This is the report" is the main clause, and "that I filed before reading" is describing the report, using "that" to indicate the whole part after is an adjective.

In the second, "I filed the report" is the main clause, but "before reading" alone isn't really grammatical to describe something when it comes after a main clause. When used this way, it feels like the "reading" has to be directly describing another word, since it doesn't have a word like "that" or "which" to say that it is describing another part of the sentence. Since it sounds incomplete in this way, it can be unclear WHAT you didn't read before you filed the report. Guidelines for how to file a report? The report itself? A notice that says not to file a report that day? It's missing something.

There's probably more technical terms for these, but I don't know them. I guess the teacher is right that it's hard to explain for a native speaker why it's wrong, but I know it sounds wrong to me.