I don't think this logic is nearly as clearly-cut as you imply it is. First, from what I remember from talks by pain experts, the body does not always send accurate pain signals. "Pain" from an amputated limb is one example, so masking invalid pain would be a good thing, not a bad thing if that is happening. Second, if people were continuing to do damage due to pain masking, we would expect to see rampant acceleration of joint damage (especially tendon ruptures) in people using N who previously had joint pain. While I don't know of a way to objectively track that, it certainly isn't reported on forums like this and just the opposite seems to be true. Third, there are a lot of positives to enabling more activity (blood flow, glycogen uptake, myokines, cardio conditioning etc.) that may partially or fully offset any masking. And finally, Nandrolone is apparently a promoter of tissue/collagen deposition, which would be a legitimate improvement, not masking.
That said, a partial confounder is the generally joint-unfriendly form used by most lifters in most gyms. Examples include bouncing bench presses, leg-presses with too much weight and too little range of motion, uneven training (too much pushing, too little pulling), over-training, too much volume at absolute limits, training through pain, etc. If the Nandrolone is enabling that, then the results would be bad, but that is not the fault of Nandrolone.