If you add the Comment field on any Issue Views on GH the field shows the latest comment but it doesn't inherit the comment visibility from Jira.
This misbehaviour happens on Planning board and Task board with any GH views (Summaries, Cards and Lists).
Steps to Reproduce:
Add the comment field to any Issue View on GH;
Create a issue and add a comment setting any visibility restriction;
Go to GH (Planning Board or Task Board) and any user will be able to the comment;
Go to Jira with the user that doesn't have the permission to see the comment and he will not be able see it. Only on GH view.
I have attached some screenshots examples.