In a support thread titled ‘Full Name of user not shown in posts,’ Mikael_Baggström reports that despite enabling ‘display name on posts,’ some users only show their username, not their full name. michaeld suggests this occurs when the username closely resembles the full name, noting the same behavior applies to Mikael’s own account on Meta. Mikael_Baggström confirms this but seeks a way to always display both username and full name, even if similar, to promote transparency. JimPas clarifies that users may leave their full name blank or incomplete unless it’s a required field. codinghorror confirms it’s not possible to display redundant info when names match. Mikael_Baggström points out that usernames like ‘medhathanbali’ lack spaces and obscure real names, asking how to prioritize full names in such cases. codinghorror explains the ‘prioritize username’ setting is on by default and notes Mikael is seeing its effect on Meta. Mikael_Baggström confirms finding and resolving the issue. The thread concludes that while redundancy can’t be forced, adjusting the ‘prioritize username’ setting allows full names to appear where they differ from usernames.
I have the turned on the setting “display name on posts: Show a user’s full name on their posts in addition to their @username”.
It works, but not on all users. I have found a user that I can only see his username, not the full name in posts.
Is this a setting that can be overridden by the user?
PS. I want my community to be based on openness/transparency so I don’t like any user to hide his/her name.
This happens when the username is very similar to the full name, could that be the case?
The same is happening with your own user here on meta (although meta shows the full name in that case)
Ah yes that must be it, thank you. However, I would like to change that behavior, if it is possible in admin settings somewhere?
I rather show the full name than only the username, but I would prefer to always show both even if they are similar.
If a user leaves their “full name” blank, then the only thing you’ll see is the username. Some users may only fill in their first name, some full name. Unless you make it a required field, not everyone will comply. 
Not possible to display that redundant info when name and username match, no.
Well, the username is without space, so it is not clear what his name is:
Username: medhathanbali
If “redundancy” is not possible. How then can I show their full name instead of username in these cases when they “match”?
I believe the setting is called “prioritize username” and it is on by default, you are seeing the effect of changing that site setting right here on meta, look at your post
Ah perfect, found it! 