Some Qt5 ui file does not compile with Qt6

Strangely an existing simple ui application which compiles good with Qt5 does not compiles with Qt6 stating connected slot is not available. Generated ui_xxx.h for some reason tries to connect slot in parent class instead of child.

Attached ui file can be used to test it

1: download unzip and qmake make using command prompt using Qt5 qmake. it compiles well.

2: qmake and make unzipped code using Qt6.2.1 it complains 
uic/ui_guicoorddlg.h:141:94: error: no member named 'formDone' in 'QDialog'
QObject::connect(buttonOk, &QPushButton;::clicked, guicoorddlg, qOverload<>(&QDialog;::formDone));