Interrupt never fires
As you probably know, the XMEGA has a multilevel interrupt controller. You not only have to enable the global interrupt but also the desired Level-Interrupt (Low-Level since you  configured your timer priority to &quot;low&quot;) in the control register.

CTRL – Control register
Bit 0 – LOLVLEN: Low-level Interrupt Enable(1)
When this bit is set, all low-level interrupts are enabled. If this bit is cleared, low-level interrupt requests will be ignored.

The code to enable the specific interrupt level is below.
//enable low level interrupts
PMIC.CTRL |= PMIC_LOLVLEN_bm; //0b00000001