Not sure if this is a bug or a feature.
For example, if their username is “johndoe” and their display name is “John Doe” …the “John Doe” would not display next to their username on the forum with the “display name on posts” option checked.
If the username is “johndoe” and their display name is “Something Else” …the display name would appear.
Some of the members of my forum have been adding symbols like ™ at the end of their display names to get around it. The display name is still used in emails regardless.
This started happening after I upgraded to 1.2.0.beta4 from 1.0 and is still present in 1.2.0.beta5.