Somethings wrong there; as n3ds 2.1 sleep is a mcu [or some other hardware] brick, and not recoverable by hardmod [nand-restore].I'm really sorry to hear about what happened about getting your 3DS bricked.
On the technical side of things, how that old firmware bricks your 3DS is actually because firmwares that are older than 8.1 only have an old 3DS variant, there is no New 3DS version of it. And because the New 3DS is different hardware, it uses different instruction sets for sleep mode, turning off the display and many other functions.
Using a firmware that is coded for old 3DS will always result in that same problem. When the device's lid is closed, the New 3DS uses the ARM9 instruction to the NATIVE_FIRM, and what it points to is a pointer that leads to an instruction that rewrites bytes in the invalid locations on the NAND, which ends up corrupting the system NAND. And that's how it's game over for your working 3DS. The only way to fix the problem, is through a hard-mod. Only attempt a hard-mod if you know what you're doing.
Likely if above is right it causes a piece of hardware to burn out or trip