Can find the custom attribute on the return parameter of the delegate fine in Editor, but not in IL2CPP builds (tested Windows x64). Looking at the stripped C# assemblies, it appears that the attribute was not stripped out.

Steps to reproduce:

1. Build attached reproduction project with IL2CPP backend

2. Run the built application

3. Check "player.log"

Actual results: 

{{Remote return type: __ret}}

Expected results: 

{{Remote return type: InputPromptResponse}}

Reproducible with versions: 

6000.0.0b15
2023.3.0b10
2023.2.18f1
2022.3.15f1
2021.3.37f1

Tested on (OS): 

Windows 10