Momentum Unity M1E PV:01 will not take firmware beyond V2.3

[BMcGregorSr]

We have a few Momentum  Unity 171CBU98090 processors that are PV:01 that we would like to load with Ecostruxure Control Expert V16. I've tried updating the firmware to version 2.6, but it won't take. When using Unity Loader, It says firmware update successful, but stays at the original firmware version at the start of the process. When using EADM, the message after the firmware download sequence is; Firmware update failed. Version on device is different than package version and Load sequence failed. Firmware versions 2.3 and earlier are the only versions they will take.

Is there something I'm missing in the process?

We have other Momentums that are >PV:07 and they update to V2.8 without issue. I have not found any documentation that states such limitations for PV:01 Momentums.