Drive is having one of its bad hair weeks. They happen from time to time.
To answer "Does it means we can't trust the API ?", well trust is a bit subjective, but we have started to add defensive coding to our apps. So rather than assume the API is working to spec, we assume it isn't and do a double check.
Eg.after an insert we are now re-fetching the inserted file to confirm that it has been correctly inserted. We also track 304 rate limit errors on inserts because sometimes, despite the exception, the file was inserted. (similar to your observation, except you're seeing 500).