IC Enable Pin not working as expected
I was able to figure this out by breaking it out onto a breadboard. Essentially, the PIC24's UART module is keeping the TX pin pulled high, which for some reason is impacting the EN pin on the TH3122 and keeping the voltage at 2.7v. The fix here is to disable the UART module before pulling EN low, then the TH3122's vreg shuts down as expected.