Facebook username as email Registration BUG

Hi guys,
JFBConnect Configuration:
- Enable Automatic Registration: No
- Auto Username Prefix: email - an image for you: photos.app.goo.gl/PMzzkrFg3YdMwDbR9
- Username Field on Registration: Generate and Hide - an image for you: photos.app.goo.gl/sMzuU6ffwPJCF1ER9

Expected behaviour: during Facebook Registration the "Username" field is no shown, but it is automatically filled copying the email field.

What's happened: During a Facebook Registration the User has been able to Register through Facebook, but the "Username" has not been filled with the email.
This is a big problem because the User does not see his username, so it doesn't know it, and on my portal the "username" label has been changed with "email" = the User after registration cannot login
What I'm describing happened on 06 june 2020 with Joomla and JFBConnect up to date, but I only realize it now through the user's complaint. So:

1 - is a "Bug" already known and yet solved?
2 - Please, Can you check ? Are you able to reproduce it?

Hi Melissa,
"...Have you seen this behavior again?" = No, but it is not so common for me to have registered users directly from Facebook, this is why I asked.

With the goal to make some tests, if I make a copy of my live site, Is there a way to setup Socials test APIs so as not to dirty the main site with its real socials ?
Please, Can you address me on the right way to do it ?