HI Stefan, I ran into this predicament just this week for my work as a music supervisor tasked with creating a new re-record of the tone poem and I have an answer for you.
Back then, works copyright terms began when scores were published with a copyright notice. So although the score was completed and premiered in 1928, Warner Brothers first published the sheet music in 1929 and as such the publishing rights will not enter the public domain in the United States until January 1, 2025.
This has been confirmed by our musicologist and the Duke University Center for Public Domain.