@d-healey

In case anyone else should ever run into this problem here is the solution (I'm kicking myself).

The Expansion Selector has to be higher than the sample map selector in the UI tree.
That's it.

That's why the minimal project worked, and that change seems to have completely fixed it in my main project.

Thanks again for all your help. Hopefully this will help someone in the future!

@d-healey

Thanks for your willingness to help David. I just made a dummy project for testing. Here's what happened:

Factory: 2 presets and 2 sample maps
Expansion 1: 2 presets and 2 sample maps
Expansion 2: 2 presets and 2 sample maps

Exported to VST3, created a DAW project and...

No issue. The problem is totally absent! I'm not completely certain as to why, but I suspect maybe it has something to do with where the code is in the hierarchy.

Thanks for guiding me through this, I think I have the necessary tools to get across the finish line without pestering you any further! Just in case, here is the code for the dummy plugin.

Content.getComponent("knbDum").setControlCallback(onknbDumControl);

// The sampler references
const var Sampler1 = Synth.getSampler("Sampler1");

// The ComboBoxes for Expansions
const var ExpansionSelector = Content.getComponent("ExpansionSelector");

// The ComboBoxes for SampleMaps
const var cmbSampleMap = Content.getComponent("cmbSampleMap");

// Expansions Handler Create
const var expHandler = Engine.createExpansionHandler();

// Expansion Types Allowed
expHandler.setAllowedExpansionTypes([expHandler.FileBased, 
                                     expHandler.Intermediate, 
                                     expHandler.Encrypted]);
                                     
// Expansions list retrieve
const var expansionList = expHandler.getExpansionList();


// Extract names of the expansions
const var expansionNames = [];

// Add “Factory Content” as the first option
expansionNames.push("Factory Content");

// Add all available expansions
for (e in expansionList)
{
    expansionNames.push(e.getProperties().Name);
}

// Fill the Expansion ComboBox
ExpansionSelector.set("items", expansionNames.join("\n"));

// Global variable for saving the current expansion
const var currentExpansion = -1;

// Help function for updating all SampleMap ComboBoxes
inline function updateAllSampleMapComboBoxes()
{
    // Empty all SampleMap ComboBoxes
    cmbSampleMap.set("items", "");
    
    local actualValue = ExpansionSelector.getValue() - 1;
    
    if (actualValue == 0) // "Factory Content"
    {
        // Use the embedded SampleMaps for “Factory Content”
        local maps = Sampler.getSampleMapList();
        local mapItems = maps.join("\n");
        cmbSampleMap.set("items", mapItems);
    }
    else if (actualValue > 0 && actualValue <= expansionList.length)
    {
        // For expansion, load the SampleMaps of the selected expansion
        local exp = expansionList[actualValue - 1]; // -1 because 0-based index
        
        // Set the expansion as active
        Engine.setCurrentExpansion(exp.getProperties().Name);
        
        // Get the SampleMap list of this expansion
        local maps = exp.getSampleMapList();
        local mapItems = maps.join("\n");
        
        // Set the displayed names in all ComboBoxes
        cmbSampleMap.set("items", mapItems);
    }
}

// Callback for Expansions-ComboBox
inline function onExpansionSelectorControl(component, value)
{
    updateAllSampleMapComboBoxes();
}

// Callbacks for SampleMap-ComboBoxes
inline function oncmbSampleMapControl(component, value)
{
    if (value > 0)
    {
        // Get the SampleMap name from the ComboBox
        local mapName = component.getItemText();
        
        // Load the SampleMap into Sampler1
        Sampler1.loadSampleMap(mapName);
    }
}


// Set the callbacks
ExpansionSelector.setControlCallback(onExpansionSelectorControl);
cmbSampleMap.setControlCallback(oncmbSampleMapControl);

// Initialize with Factory Content
//ExpansionSelector.setValue(1); // Sets to first entry (1-based)
//ExpansionSelector.changed();

Thanks again! If I run into more hurdles with this issue I'll return to this thread.

@d-healey Sure thing. Here it is without the IRs. Feel free to throw any old audio files in there.

Convolution -Test.zip