Member registration wrongly rejected on mismatched password

We had a problem whereby about 5% of new members’ registrations were failing because although they existed as users, their email addresses were being wrongly rejected as non-matching (even when copied and pasted). I noticed all the affected users had quite long email addresses, which they were also using as usernames. I discovered that if I deleted them as a user and then added them afresh as a member, it worked as long as I replaced their username with something shorter