I had cories for a while in the same tank as tetras (and, briefly, harlequins). They didn't fare well, unfortunately, and seemed to develop missing pieces of tails and fins (among other things). I did catch my two female tetras lurking around immediately behind the cories, clearly waiting for the cories to do their rummaging and avail of whatever they managed to dig up as often a cloud of debris cascaded behind the cories. I'm almost certain the cories' missing tails were caused by the tetras (this species has teeth) although I didn't rule out some being caused by over-amorous mating behaviour among the cories.
Fishes' behaviour does change over time in a tank and it might just be that one of the tankmates or the BN plec has realised, like my tetras did, that they can get a free helping of food from whatever the cories dig up - but in the process have been too eager and taken a mouthful of the cory's tail by accident.
I would suggest:
* increasing the number of cories to at least 6 (and ideally 8-10, if your tank capacity allows); I know our Community Creator says 4 but, generally, the more, the better and it ought to give them more confidence in the presence of the other tankmates (as well as minimise any stress-induced conditions which this might even be);
* doing more and larger water changes to keep the water in optimum condition to let the cory's tail heal (if that doesn't work or the situation deteriorates to the point of finrot, then remove the injured cory and medicate with something like eSHa 2000 or Waterlife Myxazin in a quarantine tank (if you have one plus a spare filter and heater, transferring some filter media across into the QT's spare filter).
Hope that helps.