The following piece of code adds all the modes supported by the station which is sending to the remote station:

void
MeshWifiInterfaceMac::ForwardDown (Ptr<const Packet> const_packet, Mac48Address from, Mac48Address to){

...

if (m_stationManager->IsBrandNew (to))
    {
      // in adhoc mode, we assume that every destination
      // supports all the rates we support.
      for (uint32_t i = 0; i < m_phy->GetNModes (); i++)
        {
          m_stationManager->AddSupportedMode (to, m_phy->GetMode (i));
        }
      m_stationManager->RecordDisassociated (to);
    }

...

The to variable is the address of the mesh point device which is the final destination of the packet (not the next-hop).
Should to be replaced by hdr.GetAddr1(), which is the mac address of the next-hop interface which is going to receive the packet?

Best regards,
Giovanni