The comment you can provide when working in the Implementation stage of Change ticket is not saved.
For example, when Change Implementation fails, the 'Provide Comment' field is the logical place to provide reasoning or details for this (and it is also the only field on the form). When data is entered there, it is not visible in the portal later.
ServiceDesk 7.1 SP2
This issue has been fixed in ServiceDesk 7.5 MP1, 7.5 SP1 and 7.6.
Same issue (although with a different cause) reappeared in ServiceDesk 8.0 and is fixed in 8.1.