Yes, that looks like a chain skip.
the way to avoid chain skip is not with weight, but by looking at the angle of the chain. This type of chain is only designed to tolerate about 3 degrees of angle at the sprocket.
the most common cause of chain skip is that the motors are too far forward or back, look at the machine from the end, are the chains parallel to the workpiece? or are they angled? you want them as close to parallel as you reasonably can. some misalignment will work, but when it gets more than about a 1/2" per foot, it’s likely to cause grief.
This applies to the slack side of the chin as well, which is why having the slack in the center is better than having it hang down from the sides (the long part hanging don from the side, especially if there is too much weigh, pulls the chain close to vertical, which would be 15 degrees to the sprocket (a bit over 3" to the foot), see Weights for chain tension for a picture of the chains in the center with weights.
if you are sure about which side it skipped on, you can probably just move that side one link.
when you calibrated, did you mark a link before extending the chains? you really do want to do that so that you can reset the chains to known lengths and not have to recalibrate.