I am using QT 5.15.13 with MSVC 2022.
Prior to version 3.3.1.1 of QTVSADDIN this code snippet was accepted in moc:

#include <QtGlobal>
#include <QtUiPlugin/QDesignerCustomWidgetInterface>

class SplitCanvas : public QObject
                  , public QDesignerCustomWidgetInterface

{     Q_OBJECT     Q_PLUGIN_METADATA(IID "SplitCanvas" FILE "FahrtauswertungsPlugInQt.json")     Q_INTERFACES(QDesignerCustomWidgetInterface) // <---- error points to here ... }

Now I am getting:
1>moc ...\FahrtauswertungsPlugInQt.h
1> D:...\FahrtauswertungsPlugInQt.h(21): Error: Undefined interface
1>C:\...\QtMsBuild\qt_work.targets(138,5): error MSB4181: The "QtRunWork" task returned false but did not log an error.

see also https://bugreports.qt.io/browse/QTVSADDINBUG-1283

Another issue was that the version QTVSADDIN 3.4.0 was reported as 3.3.1.1 inside MSVC Extension Manager, which led to some confusion as well.