Possible BUG: Creation of RDS Proxy truncates the last value from the subnet list

Question

I started with Terraform Provider for AWS RDS Proxy and found that state was always different from the applied code, only in the case of subnets - one of the subnets I specified was missing.

So I tried to create one RDS proxy via the AWS Console UI and discovered the same problem: when I set 3 subnets, the RDS proxy was only set with 2 of them. After setting it to 4 subnets, I ended up with an RDS Proxy with 3 of them.

A proxy server was created for the RDS Aurora MySQL v2 Serverless Cluster.

Any ideas why this is happening?

Accepted Answer

Ok, I found the problem.
According to docs: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-proxy.html#rds-proxy.limitations
> Certain Regions have Availability-Zone (AZ) restrictions to consider while creating your proxy. US East (N. Virginia) Region does not support RDS Proxy in the use1-az3 Availability Zone.

One of my subnet was in use1-az3 region.

Then it would be expected the API returned an error when subnet from a forbidden zone was selected to be in use.
Currently only returns an error when 2 subnets are selected as input. With 3 or more, everything passes, but the forbidden subnet disappears.

Possible BUG: Creation of RDS Proxy truncates the last value from the subnet list

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