you can obtain the Markup information of Philippe MarkUp3D through viewer.getState () and record this information in the database. After reloading the model, you can reply by executing viewer.restoreState (viewstate) , because after loading the MarkUp3D extension, your viewState will have an attribute called Markup3D . When replying, the MarkUp3D extension will read this attribute and restore the Markup to the screen. My test code is as follows:

var viewstate = viewer.getState();
viewer.restoreState( viewstate );

data structure:

I hope it will be helpful to you

save: var info = viewer.getState ();

        var str = JSON.stringify(info);
        document.inputForm.elements["markup"].value = str;
        

Why recovery has no effect:
function onDocumentLoadSuccess (doc) {

        // A document contains references to 3D and 2D viewables.
        viewables = Autodesk.Viewing.Document.getSubItemsWithProperties(doc.getRootItem(), {'type':'geometry'}, true);
        if (viewables.length === 0){
            console.error('Document contains no viewables.');
            return;
        }
        // Choose any of the avialble viewables
        var initialViewable = viewables[0];
        var svfUrl = doc.getViewablePath(initialViewable);
        var modelOptions = {
            sharedPropertyDbPath: doc.getPropertyDbPath()
        };
        
        var viewerDiv = document.getElementById('MyViewerDiv');
        viewer = new Autodesk.Viewing.Private.GuiViewer3D(viewerDiv,{
            extensions: ['Autodesk.ADN.Viewing.Extension.BasicES2015','Viewing.Extension.Markup3D']
        });
        /**
        ,'Autodesk.Viewing.MarkupsGui' //2D
        ,'Autodesk.ADN.Viewing.Extension.PropertyListPanel' //
        ,'Autodesk.ADN.Viewing.Extension.Material'
        ,'Viewing.Extension.Markup3D' //3D
        ,'Autodesk.Billboard',
        ,'Autodesk.MyBillboardGui'
        */
        var errorCode = viewer.start();
        // Check for initialization errors.
        if (errorCode) {
            console.error('viewer.start() error - errorCode:' + errorCode);
            return;
        }
        // Choose any of the available viewables.
        indexViewable = 0;
        lmvDoc = doc;
        var markup = ${markup};
        alert(markup);
        if (markup!=null && markup != ""){
               viewer.restoreState(markup);
        // Everything is set up, load the model.
        loadModel();
    }