Before running the victory lap, we shouldn't rule out that he may not have officially been pronounced dead for a week.
Do we know the method? From my understanding its not unusual for people who OD to hang in for a few days before they finally succumb.
I popped to twitter and saw that one guy in denial, so i hope it was all a lie as that would be funny. Especially considering he said the farms would be in denial if a US citizen was on the list.
Others already covered the timing aspects of the situation which hem the situation into a rather specific corner, so I won't reiterate the timeline and who said what when.
Taking all the timeline into account, I've been mulling over the possible scenarios where Byuu is actually dead, and I don't think you are exactly wrong, it
could happen that somehow between the Japanese government and the US government Byuu didn't end up on the list this time.
Issue is, the Japanese don't tend to take filing paperwork on time lightly. The most plausible explanation is they file death certs with foreign embassies on a schedule (perhaps Friday of every week) and that missed the cutoff for the month of June. I wouldn't say this is exactly probable, the digitization of records pretty much obviates the need for a practice like that which would actually hold up records (something tells me even if the embassy wants a hardcopy, they'll accept a digital copy for reporting purposes), but this is the only explanation I can think of that doesn't involve the Japanese filing something late or incorrectly, which are alternative scenarios I'd classify as "in a universe of infinite possibilities, sure, it could happen, but I wouldn't bet a 5 dimensional meal on it".
And whether such a practice exists is checkable, I'm just not sure with whom one would check.
The more likely explanation is a fuckup on the US government's part based on pure competency comparisons. That said, when the number of US nationals in Japan that die can be counted on two hands in a given year, even with DMV employee levels of competence I think in 6 months whoever the desk jockey in Japan is can manage that workload, after all even the shittiest DMV employee gets some things done every day. And there isn't a whole lot to process from Japan. It is probably somebody's job ever 6 months to compile the list and when you're dealing with an average of 6 entries that is maybe a day of work. With frequent coffee breaks.
So I don't buy that explanation either.
I actually settle on the more outlandish explanations which still result in Byuu being dead as the most probable explanations (where Byuu isn't alive) at this point, such as "wasn't actually in Japan, but did kill himself". These of course run into issues with why he'd then get friends to tell a story where all the relevant details except him killing himself are lies, but in light of the lack of report this is one of the few options that seems to have any significant probability, it's just an increasingly vanishing probability.