You can not remove an to be unpublish document from a release. Seem like a bug. seem link below for details
https://share.getcloudapp.com/ApuAnrRO
Thanks for point this out @feng.cheng, this does indeed seem like a bug.
I've passed this to the @Prismic-Dev-Team to investigate.
Hi Phil, any update on this issue?
Hey Feng,
Nothing yet sorry. I’ll let you know as soon as I know more.
Just an update this fix is currently in the 'review and testing' phase once it has been release we'll update you here.