I've seen pictures from that manga/anime before but never actually knew what it was called... Always way behind on these things, but now a couple episodes into the anime and I think I'm a fan as well.
Anyway, to follow up on a couple other points - I think overthinking the safe code may have been part of the issue, as Gatr mentioned. I think maybe people more familiar with puzzle game/escape room type tasks may have been expecting another piece of the puzzle rather than just like, counting up, which might have been why your tester was able to get it more quickly (or not! People's minds work in mysterious ways). It's always tough to get puzzle balance right, especially in a jam when testing time is so limited, so I don't really hold it against you or anything.
Regarding the test SFX, I have seen it, and I didn't mean to indicate that I hadn't - my point was more that it was uncommon for jam entries (out of the 60-70 I've played so far this year between Novembuck and May Wolf I can probably count on one hand the ones that had it) and I just wanted to praise the attention to small QoL details like that.