Looking at the history of a user's talk and entering "discretionary sanctions alert" for the Tag filter shows an extract from the history.
Clicking the "prev" diff in any line should show a difference of the alert being added. Currently, however, the diff is meaningless because it gives the difference between successive entries in the filtered history. In this example, the first two entries are dated 5 August 2018 and 6 July 2018 because an alert was added on each of those dates. However, the prev diff on the first line gives the difference from 6 July 2018 to 5 August 2018 which is no use.
I mentioned this at en:WP:VPT. I think that the prev diff worked in the past.