I have also recently deploy the latest version of SolidCP with SimpleDNS Plus 8 and ran into the same issue.

When you create the domain in SolidCP it creates a NS record using the host name configured in SimpleDNS Plus. Look in your Options, General settings for this.

If you set the primary and secondary DNS records in the Server, DNS, Service Properties using the same NS server as configured in SimpleDNS Plus then it will not add any of these records when provisioning the domain. If you remove the NS record from the Service Properties that matches the SimpleDNS Plus host name setting, then it will provision correctly.

Not sure why it automatically adds this NS record in by default but it seems to be a SimpleDNS Plus doing it. If you create a primary zone in the SimpleDNS Plus GUI it does the same thing and automatically adds itself as a NS server.

SolidCP is obviously not checking whether the records are provisioned correctly when provisioning and just failing silently.