Based on limited studies, PVC can degrade underwater due to chemical and bio-microbe attacks, it can lose weight and become more fragile after an extended time. But under the same attacks, the cement can also have degraded and lost the bond with the PVC water stop. It looks like this is the case as the photo is shown - a stretch of watermarks tracing the construction joint.
Both the design and construction methods may share the blame for water intrusion.
Design didn't incorporate water-tight requirement which was developed during the 80s and 90s. The requirement includes an increased amount of reinforcement to reduce concrete stress, and an increased thickness of the concrete clear cover to the reinforcement, both are in efforts to minimize the occurrence of crack, the spacing, and the width of the crack. Also, the resulting thicker wall lengthens the path for water migration.
Construction care. Since the water stop is much lighter than the concrete in the fluid state, it is difficult to maintain the water stop in the proper position, and resulting in locations with an inadequate cover thickness that shorten the water flow path. Also, under-vibration and over-vibration both can cause dislocation and poor bonding.
However, after the structure has stood for 50 years, I wouldn't put too much blames on the design and construction. For leakage problem caused by which tends to occur shortly after the construction, and the problem tends to be widely spread, and can only get worse with time.
IMO, any localized repair will not be durable. Water will find its way to get in if the repair does not correct the problems that may be present somewhere away from the leakage. If accessible, waterproofing the exterior wall face is the most effective, but costly, method. Otherwise, a false wall with a proper drainage/dewatering/ventilation system may be considered.