UnityCloudBuild: Codesigned with Sandbox enabled, but still get ITMS-90296: App sandbox not enabled

Question

I've made an MacOS app with Unity Cloud Build and I want to sign and distribute it using App Store Connect.

I download the compiled .app file and use codesign to sign all the appropriate files. I also use an entitlements file when I sign the runtime binary.

I used the command codesign -d --entitlements on the resulting .app file to confirm that com.apple.security.app-sandbox is set to true, which it is.

But when I use productbuild to create the .pkg file and upload it using Transporter, I get an e-mail from App Store Connect saying that "ITMS-90296: App sandbox not enabled"

I don't know how to further debug this... Does anyone have any pointers on how to fix this?

Note: it has to be doable either via the Unity Editor, Unity Build Cloud or the MacOS CLI...

MacOs CLI Codesign showing that app-sandbox is enabled:

Mac-mini:Downloads mini1$ codesign -d --entitlements :- MyApp.app
Executable=/Users/mini1/Downloads/MyApp.app/Contents/MacOS/MyApp
warning: Specifying ':' in the path is deprecated and will not work in a future release
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "https://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>com.apple.security.app-sandbox</key>
<true/>
<key>com.apple.security.cs.allow-unsigned-executable-memory</key>
<true/>
<key>com.apple.security.cs.disable-library-validation</key>
<true/>
<key>com.apple.security.network.client</key>
<true/>
</dict>
</plist>

(added formatting, Note the com.apple.security.app-Sandbox key and value)

The error from App Store Connect:

ITMS-90296: App sandbox not enabled - The following executables must include the 'com.apple.security.app-sandbox' entitlement with a Boolean value of true in the entitlements property list: [[myapp.pkg/Payload/myapp.app/Contents/MacOS/myapp]] Refer to App Sandbox page at https://developer.apple.com/documentation/security/app_sandbox for more information on sandboxing your app.