...
The mentioned above message "Team statistics are disabled in configuration. You will be able to see only your own upcoming vacations" does not affect team managers nor HR members as in the presented case, but only individuals from particular team. It aims at protecting privacy of each one in regards to approved leaves. It can be changed by Jira administrator as presented below, under (1):
-%20enabling%20Team%20statistics%20in%20configuration.png?version=1&modificationDate=1549377871545&cacheVersion=1&api=v2)
Upon selecting "Yes" and saving settings, under "Allow users to view statistics", employees can see all approved documents of their teammates.
...