Steps to reproduce:

    *    Unfortunately * there is no a guaranteed way to reproduce the problem.
    I noticed that the problem most often occurs when the server generates multiple alerts (4 or more) of the same trigger at the same time. In this case one, or two events will be without tags from slack webhook.

Result:
See screenshot...
Event ID = 949792

See log file...
I've added to the Slack webhook line to get debug of tags for each events, to be sure that slack webhook always return always proper tags:

Zabbix.Log(3, '[ EventID = ' + params.event_id + '] Tags : ' + JSON.stringify(result.tags));

As result, I see in zabbix server logs information about triggered event from screenshot:

# grep 949792 /var/log/zabbix/zabbix_server.log
 64205:20210225:025636.882 [ EventID = 949792] Tags : {"__message_ts":"1614218196.043000","__channel_id":"CEZ2QE43U","__channel_name":"emergency","__message_link":"https://kwebbl.slack.com/archives/CEZ2QE43U/p1614218196043000"}

Slack webhook send correctly tags in all events, but zabbix server don't add tags to the event in some cases.

Expected:
See screenshot....

See attached patch file...