"The TSTMR runs off the 1 MHz clock and resets on every system reset. "
What 1 MHz clock? Do not see anything about it in the clock tool.
There are a couple of vague references in the rest of the RM (Table 5-1,54-1, ) that seems to imply that the 1 MHz clock comes from SIR/8. Nothing explains what happens to TSTMR if SIR is turned off to save power.