Unsure if this is what is happening (can't look through code / don't know what I'm doing) but i believe Subspace Bubble is achieved when your entry point and exit point are +=2000 blocks away from eachother, in any direction.
Verified in 1.21, a friend in a server of mine killed about 18 Elder Guardians with Looting 3, with zero trims.
Confirmed for 1.21.1