SQL Server 2017 CU3 will not add read-scale replica. Thinks there is a cluster Using SQL Server 2017, CU3. I successfully create a 2 node cluster=NONE availability group. Both replicas are VMs local to the site. When attempting to add another replica that is off-site, I get sql error 19417, which basically says my replica add failed because the operation is not supported when the AG is running under a Windows Server Failover Clustering context. However, I am not using a cluster, that is the point of the cluster=none AG. I have had trouble before with CU1, and went backwards to the RTM version. That is my next step. Posting in case someone has seen something similar.
I finally found Creating a clusterless availability group using windows server 2016 and sql server vNext CTP 1.4 on Stack Overflow, which got me on the correct path.

I had been using the wizard to create the AG, when that failed I used the wizard to script out the t-sql and run step by step. What I failed to notice is that the wizard creates the replica on the primary, but never actually joins the replica on the secondary.

That is, this command executes on the primary:

ALTER AVAILABILITY GROUP [XXXXX]
ADD REPLICA ON N'YYYYY' WITH (other stuff)


but this command never runs on the new secondary:

ALTER AVAILABILITY GROUP [XXXXX] JOIN WITH (CLUSTER_TYPE = NONE);


And apparently the cluster_type = none is critical. At any rate, I am watching my clusterless AG successfully build now, so woohoo! And maybe worth mentioning that I am using SSMS 17.5. The wizard code seems to need an update to include the missing step. 

As a follow up. I worked with MS through my company's support agreement. They verified there were a few scenarios that exhibited the wizard behavior described above. They are targeting SSMS 17.6 to incorporate the updates.