I know the move (back+LP+BL+LK), but I've only been able to pull it off once. Is this a known glitch or am I doing it wrong? Is it anal about timing? Btw, using JoyToKey with xb360 controller, but tried on keyboard and it's the same thing.

the correct sequence is B+LP+LK
and you have to press it REALLY simultaneously and rapidly, like a tap

Similar issue, but in Mortal Kombat Trilogy (GOG):

For the player starting LEFT side, can perform Sub-Zero Slide (B + LP + BL + LK) from RIGHT to LEFT easily without any problems (press and hold the key combination, no tapping required), but it's nigh impossible to pull it off from LEFT to RIGHT. It can be done, just may happen once, while tapping numerous times. For the player starting RIGHT, couldn't perform slide from any side at all. Surely not normal.