MSPM0-SDK: Code Composer Studio Academy Lab, cannot get hardware interrupts working

Part Number: MSPM0-SDK

I'm a newbee running the labs in "MSPM0 Fundamentals Workshop with Code Composer Studio".

Builds & runs great. Now I try to add interrupt capability with the code below:

void GROUP1_IRQHandler(void)
{
    switch (DL_Interrupt_getPendingGroup(DL_INTERRUPT_GROUP_1)) {
        case GPIO_BUTTONS_INT_IIDX:
            if (led_on_off_cycles == 312500) { /* Reset to original frequency when limit reached */
                led_on_off_cycles = 10000000;
            } else if (led_on_off_cycles >= 625000) { /* Double LED frequency: LED delay currently >= 625000 cycles; can halve */
                led_on_off_cycles /= 2;
            }
            else { /* limit LED cycles to 312500 min */
                led_on_off_cycles = 312500;
            }
            /* Toggle LED4 */
            DL_GPIO_togglePins(GPIO_LEDS_PORT, GPIO_LEDS_USER_LED_4_PIN);
            break;
    }
}

Compiles fine, no complaints but interrupts via the pushbutton on my launchpad (LP-MSPM0G3519) are ignored.
Verified the hardware with a scope - pushbutton behaves correctly. No change in variables, no breakpoint hits.
I can manually set the "led_on_off_cycles" value, which also behaves correctly.