MSP430F5529 on the MSPEXP430F5529LP: UART is not actually transmitting despite seemingly correct setup. What is wrong?

Question

#include <msp430.h> 

static int next = 0;

#pragma vector=USCI_A0_VECTOR
interrupt void bc_uart_irq(void) {

    int transmit_flag = UCA0IFG & UCTXIFG;
    if (transmit_flag && next == 0) {
        next = 1;
        UCA0TXBUF = 0x01; // should clear UCTXIFG
    } else if (transmit_flag) {
        next = 0;
        UCA0TXBUF = 0x55; // should clear UCTXIFG (doesnt!)
    }
}

/**
 * main.c
 */
int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;   // stop watchdog timer
    
    // Setup UART pins
    P3SEL |= 0x18; // default UCA0TXD/UCA0RXD on P3.3/4
    P3DIR |= BIT3 + BIT4;

    // source TA0 from SMCLK for debugging
    TA0CTL = 0x0220;

    UCA0CTL1 |= UCSWRST;
    // source USCIA0 from SMCLK
    UCA0CTL1 |= UCSSEL__SMCLK;
    // set USCI module to UART mode
    UCA0CTL0 &= ~UCSYNC;
    // configure UART
    UCA0CTL0 &= ~UCPEN; // no parity bit
    UCA0CTL0 &= ~UC7BIT; // 8-bit data
    UCA0CTL0 &= ~UCMSB; // LSB first
    UCA0CTL0 &= ~UCMODE0; // UART mode (00)
    UCA0CTL0 &= ~UCMODE1;
    // Set baud rate to 9600 (br = 104, brs = 1, brf = 0)
    UCA0BR0 = 107;
    UCA0BR1 = 0;
    UCA0MCTL = 0x02; // UCBRSx = 1
    UCA0CTL1 &= ~UCSWRST;

    UCA0IE |= 0xFFFF; // enable tx and rx interrupts
    __bis_SR_register(GIE);

    while(1) {}
    return 0;
}

I've spent a while debugging this, in all cases dumping data to the transmit buffer does not clear the flag automatically (I've also tried clearing it myself in the interrupt) and does not set UCBUSY high. I also tried using the A1 UART module instead of A0, moving the pinout to 4.4/5 (as I've seen in some other questions), different clocks (I originally had ACLK), as well as setting the UCLISTEN bit high. I'm not sure what else I can try as this seems the same as code I've seen other questions about. I've confirmed that SMCLK is running as the timer goes up with each step of the debugger; the most reasonable culprit is that the clock is simply not getting to the USCI peripheral, but I don't know if that is true nor how to test it. (I'm using CCS and the only weird thing I've gotten is that the compiler yells at me if the data memory model is not small (code is large)).