Here is what I tested in the default and custom namespace and it works.
Admin User : Creates analysis and publishes a dashboard in the QuickSight console.
Admin User : Then shares the analysis and dashboard with another Author [Manage QuickSight > Manage Assets as > Owner for both analysis and dashboard]
The Author can open the analysis and republish on the existing dashboard.
What you tested , is it similar to mine ? . If yes and it is still greyed out :
1/ Create another test analysis and dashboard and share it with the users.
2/ If problem persists after step 1 : log a support ticket.
Hello @m0ltar - Since a few months have passed from your original post I am going to archive this question. If you still need assistance with this please post a new question in the QuickSight community and link to this topic. Due to the community focus on responding to questions with 0 replies, this will ensure you are a top priority for help from one of our QuickSight experts. Thank you!