No clue, but interested in the answer.

What I do know is that EK will let people fly on standby base in J on a 2 class aircraft (e.g. DXB-CAI) which is already heavily overbooked at the time of adding pax. The passenger will not get a seat assignment and is told that changes of being seated in J are slim.

This seems fair to me. Involuntary downgrading from F (which also occurs on BA) is a shame. The frequency of this (if existent) on EK is unknown to me.