There are no "Link"/"Unlink" buttons (for linking/unlinking account) related to Smart Action in the Assistant Directory (https://assistant.google.com/ -> explore actions). They appear from time to time, but the behavior looks unstable.
Could you please clarify at what moments each of these buttons should appear?
How this issue could be fixed?
The option to link a 3rd party account with Google Assistant is only available if the Action's development team has enabled account linking.