There's very little point in debating how drives fail, anecdotally. There's plenty of opinions (volumes) on the net about this topic.
-> How hard drives fail.
I think it is ultimately about OMV not being the one that "marked" it as dead, therefore it never happened as far as it's concerned.
Let me reemphisize that OMV is a NAS "application". For the most part, when it comes to core packages like smartctl, OMV is simply "the messenger". It creates command lines for action, given to the OS, and it relays command line messages and output to the GUI. The core app (OMV) and plugins (like md) are skipping the command line drudgery and fat finger mistakes, while guiding new users toward productive ends. That's what OMV does in a nutshell. OMV helps users to make sense of Debian Linux - that's it.
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While you've provided your opinions of what's happening, you haven't provided hard information (SMART stat's, sys logs, etc.). Without sitting in front of your console, I can't even begin to explain one of the myriad of scenarios (potentially infinite) of hard drive failure or how it's being interpreted by OMV. The best thing to do (for your data) is to acknowledge that the drive has issues and remove the slightly crippled or bad hard drive from the array before it causes serious damage and move on.
Along those lines, I think you have the information needed to replace the drive.