Imagine you receive the "uploaded contacts" email:
You recently uploaded 15,000 contacts and we just wanted to let you know that they're now ready!
We also found 1,150 duplicates and 50 invalid numbers, we've attached all the invalid ones here for your convenience, feel free to fix them up and re-upload them."
But when you open the invalid numbers file, it's empty.
Empty with commas.
Take a look at the original spreadsheet.
Most likely, when you saved this as a csv, there were 50 empty rows at the bottom, however since in the file you uploaded those 50 rows still have commas, they are being processed as "contacts".
So this is why the file with '50 invalid recipients' was blank, because those rows were blank.