Entries that used to appear in the record's activity stream/formatter are somehow removed and no longer visible.
This is caused by the entries in the activity having exceeded the current OOB max value of 250 set by the system property "glide.history.max_entries".
Increase the current number of 250 to a higher number for the system property "glide.history.max_entries"
Please note that increasing this to a much higher value would result in performance degradation as the system will retrieve a huge amount of data for records that have large entries.
Entries not shown on the activity are not deleted; they are simply not retrieved per the system property. These data can be manually seen in sys_audit, sys_journal_field, and sys_email.
More information regarding the glide.history.max_entries" system property: Journal field display limits