Javascript bridge / upcall to JavaFX (via JSObject.setMember() method) breaks when moving class into a package

Question

This is similar to this question, but I believe we are encountering different issues.

Setup:

I have a Kotlin class that interfaces with a TinyMCE instance running in a JavaFX Webview. I am setting up upcalls from Javascript to JavaFX as shown below, in the TinyMCEInterface class:

inner class BridgeObject {
    fun setEditorAndSelection(ed : JSObject?) {
        editorObj = ed
        selectionObj = editorObj?.getMember("selection") as JSObject?
    }

    fun setInterfaceContent(newContent : String) {
        contentProp.value = newContent
    }
}

// after WebEngine loads successfully
val bridgeObj : BridgeObject = BridgeObject()
(webEngine.executeScript("window") as JSObject).setMember("bridgeObj", bridgeObj)

I am then calling these methods from Javascript, in TinyMCE setup:

ed.on('init', function() {
    ed.setContent(initContent)
    window.bridgeObj.setEditorAndSelection(ed)
    window.bridgeObj.setInterfaceContent(ed.getContent())
});

ed.on('Change SelectionChange', function(e) {
    window.bridgeObj.setInterfaceContent(ed.getContent())
});

Problem:

This works perfectly fine when the TinyMCEInterface class file lies in the root directory of my application (in package com.myself.app). That is, when the file structure looks like this:

com.myself.app/
│
├─ TinyMCEInterface.kt
│
├─ Main.kt

But breaks when I move TinyMCEInterface into a package (in package com.myself.app.somepackage):

com.myself.app/
│
├─ somepackage/
│  ├─ TinyMCEInterface.kt
│
├─ Main.kt

When I say "breaks", there are no errors; the calls to member functions of window.bridgeObj simply do not happen and quietly fail. I am completely bewildered as to how this can be happening.

Thanks in advance for any advice!