Drupal always creates its own URL aliases, even when I tell it not to I don't know if this is a bug in Drupal, or if there's a configuration somewhere that is causing the problem, but when I create a new page or edit an existing page, Drupal (always? often? sometimes?) generates its own URL alias, even though I've very clearly specified that it should NOT do this, and I've provided one.  The one I provide does get created, but the Drupal-generated URL alias, which is always ugly, gets used instead.  I have to manually delete the Drupal-generated alias in order for the site to work.

For instance, I just added 2 pages, "Privacy Policy" and "Terms of Use".  Even though I specified /terms and /privacy as the URLs (and yes: of course I gave them to Drupal without the initial slashes), I got stuck with /content/privacy-policy and /content/terms-of-use as the URLs for these pages.
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. :-)