I set out to inject voltage to the optocouplers on the UCBB (ref) but ended up only having to touch the positive (unpowered) wire to the positive leg to see a radically different outcome. Here is the offending circuit path to output and optocoupler. LED lights up dimly and flickers.
Here is a good output. LED lights up clearly and doesn't flicker to the naked eye at least:
I did measure the optocoupler as best I could for continuity only and saw no indication of fault. Is this still a good indication that the optocoupler is busted? If so, what is the best course of action? I assume buying a replacement IC from AliExpress will get you some sort of knockoff that may not perform as the original? I think soldering in a new chip is pretty straightforward.