If multi-site has a different domain per site, live preview crashes the system when used on a new, unsaved entry

Description

Our site has a different domain for each site:
https://www.thewriter.com/ - en-US
https://www.thewriter.co.uk/ - en-GB

Our admin is only accessible via https://www.thewriter.com/admin

We've discovered that if you create a brand new entry, with the site set to en-GB, and then click live preview, we get an internal server error. The admin then continues to bug out such as logging the user out regularly after that.

Steps to reproduce

1. assign a different domain to two multi-sites
2. access the admin from one multi-site but switch the site drop down the other inside the CMS.
3. create a new entry assigned to that other site (whose domain is not the CMS domain), and click live preview without saving

Additional info

• Craft version:Craft Pro 3.6.5.1
• PHP version:8.0.2
• Database driver & version:MySQL 8.0.20
• Plugins & versions:

apache_access  108.162.229.250 - - [09/Apr/2021:13:06:28 +0000] "POST /index.php?p=admin/actions/preview/create-token HTTP/1.1" 403 1676 "https://www.thewriter.com/admin/entries/whitepapers/19097?site=default&draftId=201&fresh=1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.90 Safari/537.36"
apache_access  108.162.229.250 - - [09/Apr/2021:13:06:27 +0000] "POST /index.php?p=admin/actions/entry-revisions/save-draft HTTP/1.1" 200 1702 "https://www.thewriter.com/admin/entries/whitepapers/19097?site=default&draftId=201&fresh=1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.90 Safari/537.36"
apache_access  108.162.229.250 - - [09/Apr/2021:13:06:28 +0000] "POST /index.php?p=admin/actions/preview/create-token HTTP/1.1" 403 1676 "https://www.thewriter.com/admin/entries/whitepapers/19097?site=default&draftId=201&fresh=1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.90 Safari/537.36"

Reproduce the failure with two multi-sites using different domains, switching the admin site, and opening live preview on a new unsaved entry. Start by tracing the admin/actions/preview/create-token and admin/actions/entry-revisions/save-draft requests against the supplied Apache logs. Done means live preview works for the unsaved entry without a 403, internal server error, or subsequent admin logout.