One customer shared that he can't add comment to the tickets closed. Sharing some background. Hopefully it can give you some clue to share your insight.
Thanks everyone!
1) the ticket was created by the customer through email channel
2) the customer was in the project "people and access", but his mail address column is blank.
3) channel access under project settings is OPEN
4) global settings of Portal access was set as Allow customers to create accounts.