How to restrict a note field to 255 characters (without switching to single line of text)

Question

I made a custom note field named "Content_Feedback" without specifying Allow Unlimited Length in document libraries

<Field
    ID="{C9C90E3A-85AB-492E-9E47-C7A520A7DD0D}"
    Name="Content_Feedback"
    DisplayName="Content Feedback"
    Type="Note"
    NumLines="8" 
    RichText="FALSE"
    Group="AXA">
</Field>

It should thus be limited to 255 characters since as far as I know, this property is responsible from changing it from a regular single line of text field definition.

The field is used by a content type which is not overriding this definition and finally by a custom list, redeclaring the field as it.

Unfortunately, it's possible to enter more than 255 characters in it. I made several tests on my dev machine ("works on my machine syndrome") back at the implementation time and anything above 255 characters was successfully & automatically trimmed. Now it's no more the case.

• Are you aware of anything in a recent CU that would have changed this (got this on both dec 2011 cu et June 2012 cu)
• Are you aware of anything that would prevent this limit ?
• Did I misunderstand the Unlimited Lenght in Document Libraries ?

Thanks for your support