Double-Update: Suddenly it's working again? I think using Incognito sessions to test might be causing the issue... This time I also used a "plus" email for more uniqueness (while using the same inbox).
EDIT: NOPE! Nvm, it was because email validation was disabled that things were working. As soon as I enabled it again, I got a different error. Which has me suspect that the bug I was experiencing earlier is a result of the following condition.
- The user creation transaction doesn't back out on SMTP failure, thereby preventing the creation of the account due to the aforementioned constraint issue.