Apparently, the used interface only supports group monitor, therefore this needs to be checked with the manufacturer of the device.
KNXnet/IP Routers connect for runtime communication between KNX IP and KNX TP1 (mask 091Ah), and KNXnet/IP Tunnelling Interfaces, like USB or RS232 interfaces.
KNXnet/IP Router shall also feature a KNXnet/IP Tunnelling Interface. But, the Busmonitor mode of the standard KNX Data Link Layer does not allow transmission, while the Busmonitor mode is active. It can of course not be that a router between KNX IP and KNX TP1 cannot send on TP1.
Hence, Busmonitor Mode in Tunnelling Interfaces in KNXnet/IP Routers shall not support Busmonitor Mode, because that would disable their TP1 transmission possibility. That is specified in the KNX Specifications and in the KNX training documentation.