EK-TM4C1294XL: Recovering a "Locked" Microcontroller sequence has a strange behavior when BOOTCFG register had been committed

Hello all,

I need to reset BOOTCFG, which is currently committed to 0x0000B012, content to its factory default value (0XFFFFFFFE). I tried to perform the sequence described in datasheet [4.3.4.3 Recovering a "Locked" Microcontroller] via LM flash programmer and I experienced a strange behaviour.

The first unlock sequence leads to get the board unreachable via ICD (from both LM flash programmer and CCS). Power off/on doesn't fix the situation; instead perform again the unlock sequence make board reachable (with BOOTCFG reset).

If I don't program the BOOTCFG register the unlock sequence doesn't lead to any board hung up.

The sequence to reproduce is:

1. Write BOOTCFG register with a value (in my case 0x0000B012, PF4 triggers bootloader when low; but also other values leads to the same) by:
   1. Wrtiing FMA with 0x75100000
   2. Writing FMD with 0x0000B212
   3. Writing FMC woth 0xA4420009
2. Power off and on the board.
3. At this point check that LM Flash programmer,  or CCS, can interact with TM4 (for example by trying Get Current Values button in Other Utilities tab of LM) are still able to interact with the board.
4.   Perform the Unlock sequence from LM, same button shown in the picture.
5. Repeat step 3, now board is unreachable.
6. Repeat step 4.
7. Perfrom step3, the board has become reachable again.

I wonder what status TM4C1294NCPDT is in when unreachable. 

Best regards.

Lorenzo.