Would an IAM token without X-Amz-Security-Token work?

[bow-fujita]

Hi,
I came across to this old issue because my app had intermittently failed with IAM authentication to RDS Proxy due to missing the X-Amz-Security-Token parameter. The issue was closed as it started working by executing aws sts get-caller-identity and it was said "The tokens are still missing the field X-Amz-Security-Token but still seem to work fine without it".
I get confused because the IAM API reference is saying that X-Amz-Security-Token must be included while using temporary security credentials from AWS STS. Can anyone elaborate how an IAM token without X-Amz-Security-Token generated by aws rds generate-db-auth-token would be authenticated and how the GetCallerIdentity would help?