Currently When moving JIRA and Confluence to a different environment, most of the time in order to test. The new environment has different URLs, which affects the issue link in JIRA and page links in Confluence as they rely on application link which has been defined earlier on the older environment.
Confluence links in JIRA need to be updated as the new applink is created.
This issue could be solved by taking the steps at Application Link Issue When Moving JIRA and Confluence. However, this may not be applicable to Cloud since a full restore of Confluence is not possible when also JIRA is present on the same instance. Therefore the page-ids may be different as well, not only the hostname, and changing only Confluence url in the backup file won't solve the issue.
In this case you contact Atlassian support by logging a request in support.atlassian.com and ask them to apply the workaround for this issue.