Classes with C++11 style attributes confuse lupdate. A minimal sample is:

#include <QObject>

class [[maybe_unused]] Test : public Q_OBJECT {
  Q_OBJECT

public:
  Test();
};

Test::Test() {
  setObjectName(tr("Test"));
}

Running lupdate on this reports the following error:

$ ~/Qt/6.9.2/macos/bin/lupdate test.cpp
lupdate warning: no TS files specified. Only diagnostics will be produced.
test.cpp:11: Qualifying with unknown namespace/class ::Test

Removing the attribute fixes the issue.