TestFlight warning when uploading macOS app via Transporter

I'm attempting to upload an updated version of our macOS app for distribution via the App Store. We've done this without issue before, but I am now receiving a warning when I upload the app via Transporter:

"Cannot be used with TestFlight because the signature for the bundle at “AXON Studio.app” is missing an application identifier but has an application identifier in the provisioning profile for the bundle. Bundles with application identifiers in the provisioning profile are expected to have the same identifier signed into the bundle in order to be eligible for TestFlight." (90886)

I just recently started seeing this warning when I upload our application via Transporter. Before this warning started happening, I was using the exact same process and scripts to build/package/codesign our application. NOTE: we are not using Xcode to build our application, so we can't take advantage of any codesigning/packaging automation provided by Xcode (the app is written in C#/.NET 6.0), so we are doing all build/package/codesign steps using the appropriate macOS command line utilities. Also, I have verified that the app bundle and its contents have valid signatures.

Does anyone have any idea what may have changed to cause this warning, or how I might go about determining the root cause so I can fix it?