I wanted to secure my S3 bucket with allowing accesses from dynos in my Private Space with allowing only Stable Outbound IP addresses. Still, dynos don't seem to have access to my S3 bucket.
Please use the VPC ID for the Private Space which is available through the Heroku CLI command:
heroku spaces:peering:info --space <space-name>
For securing the platform, Heroku creates VPC Endpoints against S3 buckets in the same region. This affects the routing of network connections from dynos in the Private Space to your S3 bucket.