As far as I know they should all be copyright free. However, I have heard and had a similar problem myself with GarageBand loops and iMovie sound effects. Because Apple does not place a copyright on the audio clips and music, some people will register them as their own, and the falsely submit copyright reports.
I will look more into this but as far as I know, everything there should be copyright free, that is most likely an illegitimate copyright strike.
I did also find another support thread from back in 2014 regarding the music in iMovie. Though this is an older thread, I think that the same royalty free notice should still apply to the modern versions of Apple's apps. Is iMovie music copyright free - Apple Community
I will keep looking for answers, but I hope this helped to some extend!