That is where the head was welded to the socket, or in other words the weld broke. Historically the pole arm/spear head and socket were either A) forged from a single piece or B) less often the head was forge welded to the socket. Either way is different from how this spear head was made. Forge welding is a different animal from modern electric welding or acyetelene brazing/welding. In forge welding you are taking iron of steel to a nearly liquid state in an intense forge heat and and using hammer force welding the iron or steel to itself. In electric or acytelene welding your using another metal ( often a bit harder than the steel its bonding ) to join the two pieces. With modern welding if somethings going to break its going to be at the weld. Modern welds are very, very strong and do not often break but are still not the same as forge welds bonding steel to itself on a molecular and homgeneous level ( no intermediary metal as a bond ).
If you've recently purchased this and have retained proof of purchase see if the point of purchase will replace or refund. I don't know if the spear is an MRL but to the best of my knowledge they would replace pretty much no questions asked or at least used to.
