This will occur when your device is syncing to an Exchange server connected to your Outlook if you have the following configuration-

In the Informant settings, sync accounts, under the reminders slider (which should be enabled) there is a setting Import Reminders as Task.  If this is enabled it is causing the issue that you are having, to avoid this please disable this option.

When the option is enabled it pulls the task (reminder) from the iOS Reminders (Exchange) database and moves the item into the Informant database.  This allows the task to then be synced by any of the Informant sync services but it will also remove the task from Outlook as it has literally been removed from one database and put in another that is not syncing to Outlook.

This will result in the appearance that syncing with Informant is causing the tasks to be lost from Outlook but still appear in Informant.