"Last public comment date" custom field is updated by internal comments

Summary

Internal comments are updating the custom field "Last public comment date". This should happen only when adding public comments.

Steps to Reproduce

1. Create "Last public comment date" custom field type (enabled by JIRA Toolkit Plugin).
2. Create a ticket.
3. Type a comment and click "Comment Internally"

Expected Results

The internal comment is added on the ticket and the custom field not updated.

Actual Results

The custom field is updated with the date that an internal comment was added.