More Info.
If the dynamic curves are alembic cached, then they will indeed update the current xgen discription correctly, so it appears that it is live mode which only works correctly on the first made description in the scene. Any other descriptions in live mode for attached dynamic curves do not update in the viewport.
Hello,
Thank you for posting to the Area forums. I understand that you are experiencing issues when attaching a hair system onto multiple xgen descriptions.
I have been running tests on my end and have been able to replicate the issue as well. I have reported this to the development team so that they are aware and can work on a resolution.
Good thing that you are able to work around the issue by creating alembic cache of the curves.
Cheers,