SIM5360 SPI communication
It sounds like padding, since the max. transfer size is 4 bytes. I assume you mean that the receiving side sees 0x000000bb when you send data bb. It may be that you transfer unit is set to 32 bits i.e 4 bytes. You can check with AT+ CSPISETPARA?.

If that's the case, you can try changing it with AT+ CSPISETPARA =8,0,1 which means you'll only send 8 bits when transferring byte-sized data.