I did some more searching and found that the issue in our case was in the registry on the machines themselves. Although we are using Office 365, this link discussing Office 2016 worked. https://admx.help/?Category=Office2016&Policy=office16.Office.Microsoft.Policies.Windows::L_DisableO... This link is discussing how to block the Office Store, but we found that the registry setting it discusses was entirely missing from the registry for our machines, and the default setting is to allow the Office Store, so all we had to do was to create the registry setting. Oddly, although we are using Office 365, creating the registry for Office 16 worked fine.
This had to be done in concert with the Centralized Deployment discussed in the Microsoft documentation (https://docs.microsoft.com/en-us/microsoft-365/admin/manage/centralized-deployment-of-add-ins?view=o...). Once the add-in was deployed through Centralized Deployment (in the admin center) and the registry was updated, the add-in worked as desired.
I further found that once the registry setting had been created on my machine, I could export it as an .exe and then use that to update the registry on my user's machines. Fortunately, in our environment this is possible - I imagine many other environments would not allow such registry editing.
Hopefully if someone else comes across this issue this will assist them.
