Continuing to troubleshoot my 3-out-of-4 non-pairing units. I took the advice of @erktrek, and removed/re-paired the original successfully working device ("#1").
I removed #1 from the Hub, then I tried to pair devices #2, #3, and #4. No success on pairing... just the very consistent "Found Zigbee device... initializing ...", then hangs there indefinitely.
I then tried to pair #1, and it paired up successfully, immediately. I then tried, and failed, to pair #2-#4.
However, I noted physical differences in the three non-pairable units vs. #1.
For #1, the printing on the back cover is light grey, and the size and arrangement of the printing is slightly different than the other three. The ink used on the non-functioning units is black/ dark grey. See photo, with unit #1 indicated with a green check and partially removed adhesive strip.
Also, the reset button produces different LED blink patterns on #1 vs. the non-pairable units #2-#4.
For #2-#4, if I long-press the reset button for 6 seconds, the LED blinks rapidly for two seconds, then stops blinking. For #2-#4, if I quick-press the reset button, the unit goes into a slow blink mode of on for 1 second and off for 5 seconds, and this cycle repeats for a minute.
For #1: Regardless of whether I quick-press, or long-press the reset button for 5-9 seconds, I get the same slow blink mode that lasts for a minute. I cannot get this unit to produce the rapid two second blink that the other units exhibit with the long-press.
It definitely appears that my purchase of 4 units contains different versions of HW/FW. My concern is that my non-working units are the newer version.
Since, I got these units from AliExpress, I am doubtful I can get much help from the vendor. Does anyone know of other forums that are useful for Konke devices? Tagging some of the heavy hitters on this discussion. @aaiyar @JasonJoel Thanks!!