If the charge was the type that completly flattens out the current at the end of charge, and the charge and the battery have ~0 differential, then a drop like that would be noticably bad.
its hard to explain, but if you used a constant voltage , and waited till there was almost 0 current flow (concidered trickle topping) then a drop in 1/2 hour like that would be abnormal. the fully topped cell would be definatly at 4.18v , not just recentally had power applied to it, like what usually occurs. It is important to note differences in how its charged, because this could cause differences in the data people are referring to, vrses the cell actually taking a dive like that.
So it rested after charge, and went down to the actual rested voltage it was charged to ~4.10v , if you knew the Current rate that was occuring when it was at 4.18v , you could determine how much of that was just "hot off the charger" voltage.
so then if we assume it was charged to a rested ~4.10v, 3 weeks to the 4.0 would be the self discharge rate. of course the discharge rate would include any power that the protection curcuit used too.
one way I (here) will test UNprotected, is let them come down to Rested voltages off the charger , like your 30-60 minutes. then right after that you can assume that its charged TO that point rested. from there the self discharge itself can be determined, any influence from recent charging doesnt change your results.
So i charge it, test it rested, then test it in a few hours or a day, if the voltage is sinking fast, then the battery is not good. some will sink so fast , you can watch it drop on the meter, those are Really bad :-)
an unprotected good cell can read so minor a deviation from its RESTED charged voltage as to be off by .01-.03v after 3 weeks. a bad one more like .1-.3 in the same 3 weeks Unprotected, protection can add to the discharge, i dont know how much, or whos protection adds how much.
that is a long way of saying your 4.18 was not nessisarily a rested voltage, because it would be a lot if it was.
Your pictures are cool , shows 2 different batteries, cause the vent holes are different. (if they are both bottom protection, which it looks like)