Support for Messenger plugins on Safari 12 has been temporarily disabled by Facebook. This affects web growth tools that use Send to Messenger button or Checkbox Plugin only when the user is using Safari 12.
Facebook team is actively working on addressing this. Unfortunately, they do not have publicly available ETA on this feature.
Other Safari versions should be fully supported.
ManyChat team has filed a bug report with Facebook and you can see live updates here: https://developers.facebook.com/support/bugs/2236443336620367/
We will update this incident with further information as soon as we get more info or ETAs.