Bro, use
'Pelikan Pelifix' superglue...its gel type and imo its much much better than other superglue which is liquid type....can't remember where i bought it but i think its from Parkson departmental store
To fix it, find a very small tipped brush or better still toothpick...just apply a small amount of of gel into the tip and Sapu it on the Exposed area (the broken piece from the connector) and stick it back into the other piece of connector and make sure you tekan kuat-kuat for a few seconds then just leave for at least half a day la....that should solve your prob.
Well, if you ask whether the ryuki series suffers this problem, i would say, in my case, yea, cause so far my ryuki, knight and Ouja suffer this kind of problem which and
Ironically all their Left legs like your knee connector snapped into 2 though my Ouja survive as Ouja doesn't require any armor swappings....only ryuki and knight which i need to swapped for the Survive armors broke
I might rule it out as QC problem, cause my hibiki series did have this problem but its at the arm (elbow joint connection) but who knows maybe i'm just unlucky?

damn ya man i felt so angry... almost threw the damn thing out hahahhaa...
thanks for the solution bro... appreciate it... but i heard is a common problem for hibiki also... stupid ball joints and what angries me it cant stand coz the legs too soft LOL...