Yes, your hypothesis is most likely correct. Your disk is no longer assigned to the array, so Unraid sees it as an Unassigned Device. Since your high-speed share was only on that disk, it logically "disappears" from the array. Mini Militia App Lock
I had a similar issue once — turned out the SATA cable was fine but the power connector was slightly loose. Try swapping cables or ports; Unraid can disable parity even from a momentary dropout. Mini Militia App Lock
I have exactly the same problem on all my Unraid servers too. Every time I define a static port, it returns to UPNP after a restart or a refresh. I think it's a bug or a config crashing, it's frankly frustrating. Mini Militia App Lock