I hear a sound to know the snippet is registered, but no text appears. This only happens in the body and subject of an email draft. Snippets work fine in the email lines and in other Google Docs/Sheets. What might be the issue here?
Hi @Tony_Xiao Welcome to the forum! 
Are you using any other Chrome extensions as well that interfere in the body of Gmail drafts? Can you disable those extensions, reload the Gmail page, and then try the same snippet again?
Let me know if it works for you.
It looks like MixMax is interfering with the snippets, even though I turned off slash commands for that extension. Would you know of a workaround to be able to use both at the same time?
Sadly, we have no workaround for it as of now. We have informed MixMax about the issue, but we have not heard back from them.