I just upgraded to 2.8.0.beta6 and the header area (default theme) shows a text link in the top left corner Skip to main content and what seems to be a broken all categories link. The all categories box doesn’t appear on mobile, but the skip to main content text does.
So, to close the loop here, the default desktop stylesheet for the site above is older than the commit that is deployed, it is missing some recently added styles. Likely a caching issue, Amicable mentioned in private he has had caching issues before.
You will need to rebuild via the command line. I don’t know why this caching issue exists. I have a default implementation with very little customisation. My guess is something historically has broken the ability to clear caches containing CSS when upgrading via the UI.
Last time this happened, I didn’t rebuild via the command line and the issue cleared up on a subsequent upgrade some weeks later.