I have a plugin developed for InDesign Server CS5 that extends the script engine's app object with methods that can be called from jsx files. To add methods to the app object I have developed a class that inherits from CScriptProvider and overrides the HandleMethod method.
I'm updating the plugin to use it in InDesign Server CC2021, and for this I've created a new project with the tool Dolly from the plugin SDK 16.1.0.020. I've added the sources of the previous project and support for .NET. I've compiled in Visual Studio 2019 for Windows and get a binary with a .pln extension that the server instances recognize as a plugin.
Try launhing the InDesign with Admin priveleges. Sometimes the tlb file is not updated.
Also check the fr file definition of the method and the property on which the method is defined. Once the plugin is loaded check if the any methods/events/properties that you coded have been added to DOM or not if some are added and some not then you know where to focus.
Also check the script ID's
You can also match your code with the sample SDK code to identify what is different in your case.