Having problems with usernames. This really comes down to data source

Having problems with usernames.

This really comes down to data sources that are inconsistent in the upper/lower case of the usernames, say the Appian people tab definition of the user has the username "Username" but the data source we are getting a name from tells us it is "username".

We are trying to post a feed, or post to a feed, with the viewer/author using the "username" version. we get the error:
There is a problem with task “Post Closed Loan Feed” in the process “......”.
One or more users and/or groups specified for the Viewers input field does
not exist. Usernames: username. The system event was not sent.

the closest I can get to an easy simulation of the problem is using an expression rule:
working: user("Userame", "email")
failing: user("username", "email")
which we can not use as a test, as it does not return a null, or some other value, but totally fails:
Expression evaluation error at function 'us...

OriginalPostID-112110

OriginalPostID-112110