Public Mention does not show up in Mentions endpoint


I am a developer for an integration using Twitter Mentions API. Recently we came across this tweet Where while it is publicly accessible and I can fetch it with the tweet id, it does not show up in the Mentions API endpoint. My customer also verified that the tweet does not show up in Twitter mentions UI.

Here are my questions:

  1. What are some reasons a mention not showing up on the mentions endpoint? We also verified this tweet is not protected.
  2. Is there a way to poll for this Tweet using the REST Api?
  3. What is the recommended way to handle this?