Try to change params inside TQtDeviceContext.drawImage() (lcl/qt5/qtobjects.pas)...at the very bottom part of routine:
// smooth a bit. issue #29883
// try to set another render hint and try without any render hints.
ARenderHint := QPainter_testRenderHint(Widget, QPainterSmoothPixmapTransform);
if (QImage_format(image) = QImageFormat_ARGB32) and (flags = QtAutoColor) and
not EqualRect(LocalRect, sourceRect^) then
QPainter_setRenderHint(Widget, QPainterSmoothPixmapTransform, True);
QPainter_drawImage(Widget, PRect(@LocalRect), image, sourceRect, flags);
QPainter_setRenderHint(Widget, QPainterSmoothPixmapTransform, ARenderHint);