HendersonD, thank you very much for posting this, apparently this is still an ongoing issue (as your post was from August of 2018). I e-mailed Nimble Support and provided this information to them and they came back with the following info:
I traced that case and looked into the escalation of that case, we are still working on this issue currently.
Please do not install NCM at the moment.
We do not have a public document mentioning this problem.
I too had two installations for my customers, both had HPE DL360 Gen 10 Servers and these 8GB Dual MicroSD Flash USB Drives. Both installations also involved the installation of new Nimble Storage arrays. It didn't make sense why the bootbank and altbootbank properties were being lost upon reboots, but thanks to your post, I now know why this is happening. Upon rebooting, either manually or applying updates via Update Manager, the USB-Embedded device (shown below) would disappear, both from the ILO device inventory and subsequently from VMware (i.e. configure - storage adapters).
The first of the two installations was done a few months ago and I didn't see this post then so I opted to convert the servers to use HPE Smart Array controllers and SAS disks and I reinstalled ESXi and all is well. For my second installation, which I'm working on now and after having found your post, I've had to reload each ESXi server and not install the NCM. Removing the NCM wasn't the fix either, at least for me, as upon removal and reboot, the USB device was unavailable.
After reloading ESXi on these servers and not installing the NCM, they seem to be holding steady.