I found a strange anomaly: About 43000 user account entries are missing from the index. What's more, some of these did previously host active accounts. For example, the edit history for post 213893 shows edits by user 214627, but they're labeled as Oyashiro-sama in the list and their user page yields a "page does not exist" error (as do all the other vanished user pages).

The range of missing user entries is as follows:

(I didn't pinpoint exact user account numbers earlier than this point)
174965
182929
191337
212426 - 212428
212430 - 212438
212441 - 212443
212480 - 212498
212500
212503
212505 - 212513
212543 - 255322 (!!!)

Prior to 212426, there were single missing entries occasionally scattered through the user list, roughly one missing per several thousand registered users. But never anything even remotely close to this. (I checked all the way back to the beginning of the records - there's also several hundred user accounts missing out of the first 4300 or so, which is strange, but aside from that it only ever occurred in single instances or batches of ~less than 20.)

The surge in frequency began on April 3rd 2019 (server time), but there are still functional user accounts interspersed between the smaller missing batches throughout the 3rd, 4th, and 5th. No accounts remain in the listing who registered on April 6th - the latest before the missing ~43000 is user 212542 who registered on the 5th, and the next registered on the 7th. (This is highly unusual given how many people register per day on average; I doubt that no one tried to register on the 6th.)

Furthermore, the 12 user entries immediately following the missing ~43000 are strange. They are repeat entries for the same users and mostly lack IP address connection logs (which are visible to Moderators, and every other account I've looked at has at least one IP address stored):

255323, 255324, 255325, 255328 = kolemixi (no IP address)
255326, 255327, 255329, 255330 = SteinZero (IP address on 255326 only)
255331, 255333 = gekas (IP address on 255333 only)
255332 = Senryū (no IP address)
255334 = Senryu (no IP address)

Could there have been some sort of database error that is interfering with the website seeing these user accounts, or that assigned mismatched URLs or account numbers etc. to them?

Edit: I think April 6th was the same day that the rush of D-Mail spammers occurred. (According to the Kona moderator Discord's chatlogs, on April 6th one of the spammers who was caught and disciplined by Emiri-san was user 214592; this also falls within the missing ~43000 range)