I’m trying to display an account attribute in the notification email within the Registration Invitation action, but it seems impossible to retrieve it. I suspect this might be a bug, because I am able to retrieve the same attribute when sending an email using the standard Notification action.
Yeah — this is very likely expected behavior, not your template syntax.
The Registration Invitation action (Collaboration / NERM) sends a User Invitation-style email that’s rendered with a very limited context (basically the recipient + the registration link), not the richer “workflow notification” context you get from the standard Notification action.
That’s why “account attributes” (or other source-account context) come through empty / unavailable in the Registration Invitation email, while the same attribute works fine in a normal Notification action.
What you can do instead
Copy what you need onto the profile/identity first, then reference that in the invitation email (identity attributes are typically the only “user data” you can count on in invitation templates).
If you must use account attributes: don’t rely on Registration Invitation’s email rendering. Generate the invite/registration link, then send a Notification (or your own email step) where you pass the account attribute in the workflow data payload.