I haven’t seen quite this, but I did have one (of 6) Ring Extender v2 stop responding on power events. It finally was fixed after I pushed the button, which started the light flashing, and a refresh got things working again. 
To work around this, my power fail routine has voting, and only decides there has been a power fail if 3 of the Ring Extenders v2 show power fail. Concurrency issues (power flapping up and down by the utility during fail) made the design of the interacting rules a bit tricky. So, failure of any single Ring Extender does not signal power fail, nor prevent power fail detection.
Another issue is that the built-in driver had a broken refresh (necessary on power return because the Ring regains power before the hub reboots, causing the “return to mains” event to be lost), or at least it did about a year or so ago. Denny Page’s (@dennypage’s) driver, which I now use, doesn’t have the broken refresh issue.