@IKhitron Following the steps you described, clicking on "view diff" presented me with diff for both edits (tested in betalabs).

My steps:
(1) ET2 and ET3 edited board description of a board watched by ET1 on enwiki
(2) ET1 logs in to hewiki and sees

(3) Clicking on 'View changes', ET1 will be taken to enwiki to see both edits made by ET2 and ET3
(4) After that, the bundled notifications become unbundled and marked as read. If now ET1 clicks to "View changes", the diff will be shown only for that particular change.

In T190622#4092727, @Etonkovidova wrote:

@IKhitron Following the steps you described, clicking on "view diff" presented me with diff for both edits (tested in betalabs).

Hello, Etonkovidova. As I can see, you get exactly the same problem. Your middle screenshot shows a diff that includes only one edit, of ET3, when the edit of ET2 appears as diff base. If it works well, you could see some previous edit as diff base, and message "one edit of another user is not shown", as it always happens in version diffs.

Comparing the options for bundled notifications, I noticed that only the board description bundled notices have "View change" link:

Probably, we need to make bundled notices consistent in terms of links presented in notices. I've updated the title - the issue is not specific to crosswiki notifications.