You guys are right about the weights of an OS LA and any .46. I have several different .46's and, with muffler, they run between 16.5 & 17.5 ounces. The OS LA's are much lighter at about 10 ounces. If the plane was balanced with the flywheel, you can add a few more ounces to the 17.5, which is what I get for an Evo .46. This could account for your balance problem.

In addition, during production when building a large number of planes, it is not uncommon to occasionally get some heavy balsa for the tail. You could have a slightly heavy tail, plus the epoxy you added may have tipped the scales.

One of the things about RC building, whether a kit or an ARF is you must watch any weight added to the tail. A tail is typically 3 times longer than a nose so any little extra weight added to the tail will take 3 times as much added to the nose to compensate.

You also might consider a brass heavy hub. I have used them numerous times to correct a minor out-of-balance condition.