Searching by tag within a project you will show you issues that contain at least one event with that given tag. However, since issues always show the most current event, the event tag shown in the issue may not match the one you searched for.
To find the event that matched your tag, select the “Related Events” tab in the issue, then use the search to filter the list of events in the issue.
Selecting an issue [the first one for example] shows you the most recent event for that issue. Selecting “Related Events” will give you all the events for that particular issue
Searching under “Related Events” gives you specific events with the desired tag