I understand you are using split tunneling to access S3 buckets but you are getting an "...(AccessDenied) when calling ListObjectsV2" error. When you configure SPLIT VPN means specific traffic goes through VPN but your source IP remains private. For connection reaching VPC, it will have source IP that is private not the public IP of your VPN. To access S3 you either access it privately or publicly, if accessing publicly you have to access it via internet. Basically whitelist your public IP where you break out to internet for S3 traffic. When accessing Privately it will be through VPC Endpoints, specifically Interface VPC Endpoint to allow access to S3 but its only regional not for all .
When you got the “"...(AccessDenied) when calling ListObjectsV2" output Amazon S3 had issued the following AP Calls: ListObjectsV2, CopyObject, GetObject, and PutObject. Amazon S3 lists the source and destination to check whether the object exists and then performs the following API calls: CopyObject call for a bucket to bucket operation, GetObject for a bucket to local operation and PutObject for a local to bucket operation. The resolution if to Verify that you have the permission for s3:ListBucket on the Amazon S3 buckets that you're copying objects to or from. You must have this permission to perform ListObjectsV2 actions .
Refer to the references
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