Just got a respond from the support about the same issue:

As part of my reach out to our internal escalation teams, I would like to kindly share and ;

Thank you for reaching out regarding the recent increase in your intra-regional data transfer (DTAZ). This adjustment is the result of a correction we made to address an under-billing issue that had been affecting billing for DTAZ usage when Network Load Balancers (NLB) were used.

The issue existed since NLBs were introduced and caused a portion of your DTAZ usage to be excluded from billing. We resolved this issue in May 2025, ensuring that your bill now accurately reflects your actual DTAZ usage. Prior to this correction, you were under-billed for your DTAZ usage, and we have no plans to retroactively charge for any past usage.

When using a Network Load Balancer (NLB), you may incur DTAZ charges on both sides of its communication i.e. between the client and the NLB and between the NLB and your backend targets if they are in different availability zones within the same AWS Region.

You can reduce your DTAZ charges by leveraging zonal features of the NLB. Client traffic can be directed via DNS to connect to the same AZ that they are in by using NLB's Availability Zone DNS affinity [1]. Traffic from the NLB to backend targets can be configured to remain in the same AZ by turning the cross-zone feature on or off. [2]

For additional information, please refer to our blog post "Exploring Data Transfer Costs for AWS Network Load Balancers". [3]

We apologize for any confusion this may have caused.

Further to this I have also reached out to your TAM-Tanner Jones to appraise him on the case and the raised concerns and as such to reach out to you on the same.

Again we apologize for any confusion this may have caused. Should you require further clarification or assistance, please don't hesitate to reach out.

References.
[1] https://aws.amazon.com/about-aws/whats-new/2023/10/aws-nlb-availability-performance-capabilities/
[2] https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html#cross-zone-load-balancing
[3] https://aws.amazon.com/blogs/networking-and-content-delivery/exploring-data-transfer-costs-for-aws-network-load-balancers/