This will likely be due to the global setting on your system titled Absence Interface - Export Unauthorised Sicknesses, being set to false. This will mean that only closed sicknesses will export within the absence export.
To change this setting from false to true, so that open sicknesses also export, please follow these steps:
Navigate to Settings.
Expand the System Settings drop down.
Click the Global menu option.
Search for Absence interface - export unauthorised sicknesses in the name field and click into this.
Within the pop-up window, you should see a field titled Boolean, change this from False to True.
Click Save.
📌Note: Going forward from the change, any open sicknesses will export when the Absence Export is run.