A customer updated a ticket and the status changed to open. However, no content of the message shows up on the ticket.
Zendesk Support reads each received email and looks for the delimiter between the latest reply at the top and all the previous back-and-forth below. The delimiter is the part of an outgoing email notification that says:
##- Please type your reply above this line -##
Support only displays the text that is above that delimiter. Although, Zendesk stores the full email that was received, even though it doesn't show in the comment field.
To see the original update, hover your mouse over the original comment, then click the arrow that appears, and select View original email.
For more information about the email delimiter, see the articles: