I’ve checked the latest Nightly (91.0a1/20210602214447), Beta (90.0b1/20210601190019) and Release (89.0/20210527174632) under Windows 10 x64 and Ubuntu 16.04 LTS.
The system theme preview should be indeed updated to match the current design. So for this reason I will set the issue to New.
I what concerns the last update date mentioned on the details page of the theme, I believe it’s indicative of the date when the used profile was created.
For example, in screenshot 1, the last update date is June 3 2021, which coincides with the date I created the profile that is being used.
In screenshot 2, the last update date is May 24 2021, even though I updated the browser to the latest version just before taking the screenshot. Note that the profile was created on the 24th of May.