I've seen this happen once before and it is tricky to spot!
How do your users access the Service Catalogue? Is it from a custom-made button/link/webpage? If you are in control of the URL being used, you might find the URL includes a parameter to determine a user to put in the Requested For dropdown. If you remove this part of the URL it will fix the problem.
Thanks for your reply, that was exactly what was wrong. The link contained my user id. After removing that from the URL and saving it, the issue was solved.