I can confirm this strange behavior. Sometimes JTAG pins are damaged and there are strange voltages on them. I personaly fried at least 5 chips this way and only solution si replace IC. In some cases chip works fine, but is not posible to debug, sometimes stops working completly, one time was roasted to death (current more then 1.5A at 3.3V rail even in forced reset state). I have traced probable origin to some spikes when you connect/disconnect LPCLink (2.0 or older). Only working "solution" was to connect computer case to GND of debuged board. From this time there was no problem with this kind of damage. JTAG pins are somewhat sensitive on something.
Edit:
As mentioned here (Solved) LPC-Link 2 Apparently burning out JTAG connections I have added 2k2 resistors to JTAG data lines too. Woks fine on ~10cm long cable and can not make situation worse.