Thank you for your detailed post!
I understand that you're trying to create and merge a certificate signing request in Key Vault, but you're having issues when generating a new certificate because the "Merge Signed Request" button is greyed out. To hopefully help point you in the right direction, I'll share my findings below.
Findings:
When it comes to adding certificates in Key Vault issued by non-partnered CAs - I reproduced your issue and noticed that once my certificate is created, it's "enabled" and shows "Completed" when I navigate to the Certificate Operations pane, additionally the "Merge Signed Request" button is greyed out.
Looking at one of my older non-partnered CAs that I created, it looks like everything is working as expected with the Status showing "In Progress".
- Because this looks like a potential bug, I've reached out to the Key Vault team regarding this and will update as soon as possible.
If you have any other questions, please let me know. Thank you for your time and patience throughout this issue.