Thank you for contacting WPML support. I'd be happy to assist you further on this issue.
I just tested the constant on my test server and I can see it's working without any issues. The code implementation you have is also correct, so it seems like an issue only in your site.
As a test could you please use the constant inside the function once and see if that helps?
Further, could you please perform the below steps and share me the results?
1. Please follow the debugging information provided here: http://wpml.org/documentation/support/debugging-wpml/
2. Repeat the action of creating the problem.
3. Copy and paste the contents of debug.log (log file inside the /wp-content/ directory) here for me to see.