Qt sends the pointer event serial instead of the tablet event serial in data_device:start_drag or xdg_toplevel::move.

Both functions retrieve only the pointer serial.

https://github.com/qt/qtwayland/blob/5cdc706747feca4e62928332350f0229a9788ce8/src/client/qwaylanddatadevice.cpp#L162

https://github.com/qt/qtwayland/blob/5cdc706747feca4e62928332350f0229a9788ce8/src/plugins/shellintegration/qt-shell/qwaylandqtsurface.cpp#L192

While the correct way to obtain the serial is to call lastInputSerial, tablets only set the last input serials.

https://github.com/qt/qtwayland/blob/5cdc706747feca4e62928332350f0229a9788ce8/src/client/qwaylanddisplay.cpp#L895

https://github.com/qt/qtwayland/blob/5cdc706747feca4e62928332350f0229a9788ce8/src/client/qwaylandtabletv2.cpp#L173