I open commit on github here:
https://github.com/NodeBB/NodeBB/issues/10115
Fixed here:
https://community.nodebb.org/post/86534
https://github.com/NodeBB/NodeBB/commit/69a7634a78de51dfd50e72864fadf77a7b1ab46a
If you capitalize any letters of your email address when you register, you won't be able to login using your email address no matter how you capitalize it. This is because it will be inserted as the case you typed it in, but when it checks email addresses, it puts it to lowercase. The best way to solve this is just to puts the one you submitted to lowercase but not the one in the database. You could also insert it originally as lowercase, but then it might not display how you want it on your profile page.