Hello batchois,
If you're only having this issue when printing from QuickBooks it would be an application issue, and not with the printer. I would suggest reaching out to QuickBooks for assistance. You could try calling the support center at 1-800-821-2797 but they would not be familiar with the application.
I am having an issue where when I go to print checks from our QuickBooks server it automatically prints on both sides. I have verified in the browser that it is set to one-sided as well as in the printer itself. I am using remote desktop to access the QuickBooks instance(windows 10). My previous printer did not have duplex capabilities so I am unsure if there are more spots to check and nothing jumps out at me in the QuickBooks print settings. Any help would be appreciated