I read up on user-agents identified as bots, and then search here in the Netlify forums for “user-agent” and found this post, “User-agent blocking?”, where they mentioned pre-rendering. In that post, they state:
Prerendered content is only displayed in certain situations. Specifically it:
• will only be used for a Netlify website once prerendering is enabled on your site’s build & deploy settings page
• will only be returned in response to a web request when there is a User-Agent 32 sent with the request that needs this special content, or a special URL is used (see below for more details around manual testing and the special URL).
Reading up on Netlify’s prerendering, I then went and deselected the site’s prerendering option, leading to return of the expected behavior!
Once the Slack Event api verifies my endpoint, I can then re-enable prerendering 