When you echo is using the basic Amazon Music service even though you have set it up to use your Apple Music subscription you typically think that the Alexa App is no longer configured to use Apple Music as the default. You open the Alexa App, click on the ‘More’ tab then ‘Settings.’ You scroll down and choose ‘Music & Podcasts’ but find that ‘Default Services still says ‘Apple Music.’
It is likely that the ‘Apple Music’ Skill needs to be activated again. In the Apple Music Skill Settings –> Account Linking, Link your Apple Music account again.