With iphone 6s using ios v.10.3.3 and garageband on the phone. (I don't know what version of gb i have because i cannot find out. This information is not found under Setttings, anywhere. There is NO information anywhere in Settings for gb on my phone. It does not appear in the space where other apps appear in the Settings menu.)
I upgraded to the latest ios and the problem persists for all songs in gb. So I guess this is just an ERROR in Apple's program. I managed to work around this using the WALTR app. I plugged my phone into my macbook, opened logic pro and created a ringtone not longer than 40 seconds, exported it to itunes, found the file in Finder and changed the file extension to M4r for ringtones, and dragged the renamed file into the WALTR app and it went straight into the iphone's ringtone menu.
Download https://urluso.com/2yX9yA