@ShimizuA
Decision by "official sub" by Hidive/Sentai I'd consider as "another interpretation" instead of official, since their earliest subs were also full with errors and they didn't bother to make things consistent with existing translation choices by Bushiroad International for English version of Garupa.
Few facts that we know of:
1) Bandori franchise presented "LOCK" (explicit English word) as member of RAS for any RAS promotion materials such as merchandise.
2) Bandori official website for 2nd season presented 朝日 六花 (Asahi Rokka) and no nicknames, compared to other 4 characters that's already in RAS https://anime.bang-dream.com/2nd/character/
3) Bandori 2nd season ended without Rokka having joined RAS; she refered to her nickname back in junior school audibly as ROKKU
4) The conversation in that screenshot was about Saaya making a comment about it being "rock n' roll" then Rokka replied (audible) ROKKU as her nickname. It'd be a stretch to localize the conversation as "lock n' roll" and Rokka to reply her nickname was LOCK.
5) We saw evidence that CHU2 come up with creative names, so it's possible that she'd grant Rokka the name LOCK later on.
6) RAiSe! chapter 03 refer to Rokka's nickname as ロック (katakana, spelled Rokku) and nothing explicitly English word such as LOCK. If they really meant LOCK, why would they write ロック instead of LOCK like in promotion materials?
The possibilities are either
1) ロック is meant to be spelled Lock all along, which means we'll just revise our pages, or
2) When Rokka joins RAS in 3rd season, she will finally get new nickname with explicit spelling LOCK, which means her previous nickname wasn't LOCK.