[by code inspection]
What version of CI has this issue? Have you tried it on the current 3.0 dev? A lot of tweaks were made to form validation and form helpers, so it might've been dealt with already.
Please confirm that -- it's not very likely that someone will go through the trouble of trying to replicate the problem.
It's just an inconsistency I noticed while reading the code. It's still present in 3.0-dev.