This will be fixed in the next build.
As several users observed, this issue only impacted Teams users. When a user is a member of a Team, the inbox in the topbar served both network and team inbox items, but the page linked to in "all items" page didn't. This could be confusing for users who had only ever participated on a Team, as clicking the link would take them from a full inbox to an empty page.
A stopgap was to hide the "all items" link from Teams users until they had a full inbox with 100 items in it. The idea was that this would give the team time to build an inbox view on Stack Overflow and let seasoned network users be "grandfathered" into seeing the link in the meantime.
Unfortunately, seasoned users won't always have exactly 100 items, due to the way we collapse related inbox items. I personally have been at 94 since I started looking at this bug.
The good news is that Teams has in fact implemented their own inbox views by way of the "For you" page and moved all but one team off of the global inbox, so I just pulled out the offending line and we'll show the link to everyone again.
"all inbox items [support] or [bug]"
. – Rob Feb 5 at 10:16