Basically, JQL doesn't worry about the case for labels (mylabel == MYLABEL) but when creating an issue, the component does care about the sensitivity of the case (mylabel != MYLABEL) which is inconsistent behaviour.
1. Create a new issue
2. Give it label 'MYLABEL' all uppercase
3. Create the issue
4. Create another issue, and start typing 'MYL' to see that it suggests 'MYLABEL' as an existing label.
5. Delete this text and start typing 'myl' in lowercase and see that it wants to create a new label.
6. Don't create an issue and close the dialog.
7. Go to issue navigator
8. Type in JQL labels = 'mylabel'
9. See that the issue with label 'MYLABEL' appears