[LDS]

From what I gathered from a quick discussion with Sonnet Tech support, the HDX issue is related to the combination of the specific TB3 bridging chip and the TB3 interface card used in the TB3 version of the Echo Express III.

A TB3 Echo Express III with a downgraded TB2 interface card works fine.
A TB2 Echo Express III upgraded to a TB3 interface card works fine.

I have never looked at one closely, but I suspect the Avid TB3 enclosure is probably pretty similar in design to the latter, running an older bridging chip with the new interface card.

I noticed Sonnet recently discontinued the Echo Express III and replaced it with the 'Echo III', perhaps because of these issues. The new Echo III includes a specific "HDX Mode" that appears to limit the enclosure to PCIe 2.0 performance.