Steps to reproduce

1. Starting at any WordPress.com Personal, Premium site (or Business site with 8.0 Gutenberg plugin active).
2. Create a new page or post
3. Add a title in Greek (example: "Συνέντευξη της χρωματικής συμβούλου"
4. Check the Permalink/Slug preview field of the page and notice it's a number

Afterward:

5. Publish the page or post and see that the permalink is using the Greek title instead of the number that is shown in the editor.

What I expected

In step 4, I expect the Slug to be previewed as συνέντευξη-της-χρωματικής-συμβούλου in the editor to reflect what the permalink will show once the page or post is published.

What happened instead

Instead, in the editor, it displays a number the slug preview field, but once the page is published the permalink is actually showing the title.

Browser / OS version

Chrome/81.0.4044.138 / OS X 10.15.4 <- tested with

Screenshot / Video

On a Simple Site Free site, the editor shows the number in the slug preview field, but once published the slug is using the Greek title as shown in this video here: https://d.pr/v/2BPoMi

Other videos and tests:

• On a test site with the Gutenberg 8.0 plugin active the slug is stripped of characters and defaults to a number on my Business site. Example: https://d.pr/v/yQbhd9
• Without the Gutenberg 8.0 plugin enabled on my Business site the slug works fine: https://d.pr/v/ZsPzUs
• Also, on my self-hosted Jetpack site, it works as expected since there is no preview: https://d.pr/v/aubEJL

Context / Source

zd-2971946