I appreciate all the details in your question.

I don't see anything wrong with your code. A few thoughts that might help debug though:

Try toggling P1.4 on any EUSCI interrupt. Perhaps another interrupt is being generated and the interrupt isn't getting cleared as aresult.

Try writing some arbitrary data to the TX and RX buf to start. See if it needs something initialized to get shifted out.

UCBxTXBUF = 0xA5;
UCRXBUFx = 0xF0;

Is the master device transmitting in 10b addressing mode? Looks like this slave device is configured for 7b. It might help to post a oscilloscope capture of some bus traffic?