Drupal always creates its own URL aliases, even when I tell it not to
I've just had the same issue. I found the reason why it was behaving like that, so I thought I would post the answer here for others with the same problem.

You need to go to Path Settings --> Section "While Updating" (I don't know the exact wording in English as I have the PT version).

Here, you have to select the first option: Do nothing. Keep the old alternative path.

This should solve your question. :-)