I found what Microsoft says about it: Workflow that uses the SharePoint 2013 Workflow Platform Type contains blank values in output from the "Multiple lines of text" column
To work around this issue, disable automatic updating of the workflow status to the current stage name. To do this, follow these steps:
- Open the affected workflow in Microsoft SharePoint Designer 2013.
- On the Workflow Settings page for the workflow, clear the Automatically update the workflow status to the current stage name check box.
- Save the workflow, and then and publish it to the SharePoint website.
- Run the workflow again.
- When you clear this setting, you may want to use the Set Workflow Status action as part of the workflow to update the Workflow Status field if that appears to be necessary. Otherwise, the status won't be listed.
- You may also want to consider saving the value of the Multiple lines of text column in a workflow variable if you plan to update the list item within the workflow logic.
- In this scenario, the value from the Multiple Lines of text column will be blank if you edit the list item but don't change the Multiple Lines of Text column.