Can be reproduced with the example from QTBUG-87624.

QWaylandWindow::setVisible() calls QWaylandDisplay::flushRequests, which then will process wl_data_device events, calling QGuiApplicationPrivate::processDrag(). If that drag event causes another window to be shown, QGuiApplicationPrivate::processDrag() can get a reentrant call. This causes problems, since processDrag() is not reentrant-safe. (At a minimum it would need to test whether currentDragWindow has changed after each sendEvent().)

There is a simple workaround: Don't hide/show windows (or do other windowsystem manipulation) directly from drag events. Instead, use QMetaObject::invokeMethod with QueuedConnection.