Differences Between Cloud and Server/DC Version
There are some differences between the cloud and server/dc version.
Searching for similar issues when creating an issue is limited compared to server/dc version. It is more difficult to configure. You need to configure it from project settings and you also need to add a custom field to issue create screen, just below the summary field.
Customer portal is not supported
Configuration options are limited for the cloud version.
Cloud version can only search summary and description, DC version can be configured to search other fields.
Most of these differences are caused by technical reasons and there is nothing we can do until Atlassian provides new ways to extend functionality of Jira Cloud.