Front and rear Thunderbolt 4 ports are different? Is my Mac Studio defective?

I bought a Mac Studio refurb from Apple and wonder if it has a problem. I have an external drive (a WD Black SN850X inside an ACASIS 40Gbps M.2 NVMe SSD Enclosure). I plug the cable from the drive to one of the four mac studio's rear thndr4 connections. Blackmagic Disk Speed Test clocks it at 2900.

I shutdown, move the cable from the rear of the mac studio to one (either) of the front thndr 4 connections. Blackmagic Speed Test result is 700.

Am I misunderstanding something, or have I gotten a refurb with an issue? Any help is appreciated muchly. Thank you.

[Re-Titled by Moderator]