vault engine TTL requires RoleARN?

I'm not sure if this is a bug or not, but I was surprised to learn (and had to read the source to find out) that in a `vault_aws_engine {}` block, unless you specify the `RoleARN`, the `TTL` is ignored. I only have one AWS role configured per Vault role, so I had been omitting the AWS role ARN from the block with a non-default TTL. I added the Role ARN and now the TTL is honored. Is this intentional?