Numeric IDs only works when ONTAP can't find a valid user name match.
So in your case, if ONTAP can resolve UNIX user UID 327081207 to a valid UNIX user name (such as via name services), then it translates the name.
Since your ID domain is set to the default "defaultv4iddomain.com" value, it gets appended to the user ONTAP found.
So the string becomes firstname.lastname@example.org, which obviously won't map into whatever the client ID domain is set to.
The only time numeric ID support is used is when the numeric ID coming in can't be translated to a user name.
TR-4067 covers this in detail:
https://www.netapp.com/pdf.html?item=/media/10720-tr-4067.pdf - page 41
In your case, to fix the issue, change the NFS server option v4-id-domain to the same value coming in from the client. (ie, domain.priv)