The issue can encountered when the same journal field is updated multiple times in the same second. This can lead to the order of the journal entries being different from the order they were created in because the ordering is based only on the sys_created_on value, which has 1-second granularity.
It can also lead to a journal entry not being displayed at all if the journal field had a value when its parent record was inserted, and there was a following update to the same journal field on the same record within the same second.
This leads to the first comment being missed during the construction of the history set, which the activity stream uses, as that process finds the earliest audited changes for all journal fields on a record, and uses that information to search for entries prior to that date/time. If the earliest audited journal entry for a field has the same creation time as the journal entry created when the record was inserted then it will not match and will be excluded from the history set.
In an affected instance
Notice: The work notes that you entered during the creation of the HR case does not show in the 'Comments and Work notes' tab.
After carefully considering the severity and frequency of the issue, and risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make this decision lightly, and we apologize for any inconvenience. If you have any questions regarding this problem, contact ServiceNow Technical Support.
To attempt to avoid this issue any implementation would need to look at delaying or decoupling the addition of subsequent updates to the same journal field so that they don't occur at the exactly the same time. For example, using a workflow/flow triggered on the records insert which then adds the comment or generating a custom event from an after insert business rule which triggers a script action could also work.