Being a programmed myself, it looks like it's just a string for which there is no English customer text coded. As the United app/website is offered in many languages, the backend code uses strings like these which then get replaced in the customer's language at the time it is rendered to the customer.

The reason why there's no English text coded could be multiple:
* Actual bug like mentioned above (for example, the recognized string is NO_PREFERRED_CABIN instead of NON_...)
* New feature, forgot to add the English text
* File with English text could not be read (technical error, or file is deleted, or file was being modified at the time, or wrong permissions, ...) - probably not the case since the other strings were correctly replaced
* A situation they considered would never occur, and hence they never bothered to add the text for it.

I think, like mentioned above, that the cause is the first flight in Economy while you requested Business. You can then use your mind to replace the string yourself.

You could try changing the language and see if the text appears in the other language. But the chances are slim, since English is UA's main language. This would be more of an option if this actually happened on a Spanish or French setting and you changed it to English.