XB8 cannot upload large files via Wifi, but works fine when connected to Ethernet cable

Recently, I upgraded from the XB7 to the XB8 Gateway hoping to resolve this long-standing issue, but the issue persists. It actually seems to be worse with the newer equipment.

There was another similar post, but it has since been closed:

‎Xfi XB8 can not upload big files when on Wifi, but uploading big files works fine when connected through Ethernet cable. | Xfinity Community Forum 

If I change my WiFi to use my personal (iPhone) hotspot, I am able to upload large files. 

In response to some of the follow-up questions that Xfinity asked:

I am uploading 3D CAD drawing files and video files. With the CAD files, the issue can be with large individual files, or it can occur when I upload multiple smaller files in one transaction, where their cumulative is large. I don't have a threshold file size figured out. Generally, this happens when the upload package is a few hundred MB. 

When I upload these types of files to an internet browser-based service, I just get a generic error message, but when I upload to a cloud-hosted file storage server, I get a more detailed error message stating something to the effect of "the connection was closed by the remote host". This makes it seem more like it's a timeout issue. At the very least, it provides additional insight into the matter. 

Disconnecting from the VPN that I use does help. If you are experiencing something similar, that might be the bump that you need. But I would still really like to understand why this happens in the first place.