We are using a "Date Time" custom field for a project. We have a default date/time value set for this custom field. The default value is not displayed if you switch between projects using the project drop down menu at the top of the "Create Issue" lightbox screen. We are able to see the default custom field value if the "Create Issue" screen opens directly to this project.

Basically, if we were previously working with the project and attempt to create an issue, then we'll see the default value. In contrast, if we were working on a different project and select "Create Issue" and change the project on the create screen pop up, then we will not see the default value.