Well yeah, if you don’t truncate the password to 12 chars how will you fit the plaintext in a memory efficient fixed latin1 CHAR column that only accepts letters, numbers, and underscores
And then validate the email with a custom regex that definitely doesn’t account for all the valid syntax permutations defined by the several email-oriented RFCs
Only on mobile though, on desktop have different criteria. Perhaps give the text box an arbitrary max length of like 30 characters on sign-in but not on account creation.
I've had that before and I'm very confident the password was correct - my theory is that they'd changed how non-ASCII characters like £ were handled and their code only half recognised my password.