CPT-onomy not saving for attachments

Hello,

I have a CPT called Artists, which I’ve registered as a CPT-onomy for the default media attachments post type. However, when I assign an artist to an attachment while editing it, it appears to be assigned but vanishes when I update the attachment post. Am I doing something wrong?

Note: The same CPT is also assigned to some other post types as a CPT-onomy, and works perfectly for those.

Any help will be immensely appreciated.

https://ww.wp.xz.cn/plugins/cpt-onomies/