User Problem

For security purposes, content enclosed within HTML tags is skipped when displayed on websites. This is due to an instance where an HTML tag is initiated but not properly terminated, leading to the removal of the entire content block in web displays.

A User Group of Slack and the format is similar to an HTML tag. Opsgenie is detecting and considering that as an HTML tag.
https://api.slack.com/reference/surfaces/formatting#mentioning-groups

Suggested Solutions

Find a way/solution to make an exception for mentioning Slack groups.

Current Workarounds

You can see the whole description value in the payload