I too have not seen any other mention of a J Pack prior to me getting one. Take it with a grain of salt but the service manager noted to me that they've seen a number of E Packs come through with failures and he indicated its usually part of the HV wiring cables within the battery and not the actually the cells leading to the premature failures. I have nothing to corroborate this but found it interesting.
My early 2020 Model X's HV battery failed, and the diagnosis was a faulty wiring harness in the pack. The SC replaced the HV battery. The car originally had an "E" version battery and the replacement was a remanufactured "E" pack.
Hopefully these reman'd packs have a revised wiring harness as it seems like there is a higher fail rate on the original E packs.