I'm looking at specifically the error "no installation source found" received when trying to manually push apps to those devices.
Based on your screenshot showing the update mode as high priority, it seems the data is hosted in Asia-Pacific.
If you go to the client device, click on "missing" under the "Apps" section, and click on the specific app name, does it show an App source whose region differs from the device's physical location itself? I'm thinking there could be an issue trying to fetch it from a certain store's region it would not originally reach to, which results in a response that there's no source to install from.
Please let me know if you can modify that field, and if it changes the outcome or results in a different error.