When searching in JQL basic it doesn't allow to search issues created under all projects(created before) when the context is applied to one project.

JQL issue search is based on the custom field context. That's why when we search for issues where a custom field exists, all the issues listed with the custom fields are displayed for that particular project when the context is updated to a single project, not all issues under projects that are not applied in the custom field context.

The issues will be shown in JQL output as per the custom field context defined. Because of this, all issues are not listed after changing the context to a single project, and Jira is not able to list all issues from different projects.