Today I received a notification for reputation gain for an approved edit suggestion, and I found out that the link is relative. This means that if the achievement is on a different site in the network than the one you are on, it won't take you to the correct site, and instead it will take you to the same path in the site you are on. This results in you going to whatever page contains a question or answer with the same post ID.
Here is a screenshot to illustrate:
Note that the achievement is from The Workplace. Also note that the URL is relative. Clicking on this link caused me to go here:
https://meta.stackexchange.com/questions/131846/report-sites-that-use-stack-exchange-content-without-following-attribution-rules/191951#191951. This page happens to have the same answer ID as the question ID in the achievement notification.
It looks like this only affects reputation gains from suggested edit approvals.
However, the very next notification in my screenshot is from Software Engineering and it has a absolute URL. Maybe a regression happened in the time period between receiving those notifications. The very next notification entry in my screenshot is also for +2 rep but it's for accepting an answer. So it's not the same thing.