Fan_of_flux,
The bulk density of some powders can vary by a span of over 10% from lot-to-lot, so it isn't surprising that cartridges loaded with one lot fills the case better than another lot did. Additionally, cartridges subjected to vibration can pack their powder down some. With spherical powders like 748, the amount of packing isn't as great as it is with stick powders (the main reason they meter from a powder measure more consistently), but there is a little.
Breakdown of powder tends to cause it to get oily-looking and clump together, not get finer and looser, so breakdown is an unlikely cause of your observed effect. But, when in doubt, get a bullet puller and pull a suspect cartridge and pour the powder out onto a white sheet of paper. If it won't pour out but sticks in the case, even partially, that's sign of breakdown if the load wasn't compressed. Smell the powder for nitric acid scent (distinctive acrid odor, not to be confused with the ethyl ether solvent odor that is normal for powder). When you pour the powder into your scale's weighing pan from the paper, inspect the white paper for red dust. If you don't have clumping, acrid smell, or red dust, the powder is not experiencing gross deterioration. The scale will confirm or deny your remembered charge weight.
The next thing I would do is reload the same cartridge, lowering the charge to a minimum starting charge, and fire one round to confirm its velocity is lower than your new rounds. You need a chronograph for that, but you can also get a sense of recoil feel and look at the primer for more rounding and so on.
There is one breakdown scenario in which deterrents break down first, increasing the powder's burn rate. It is improbable you have that based on the lack of other signs, but an abundance of caution is justified with ammunition preparation. If you have a chronograph, you can compare your old load to new loads and confirm the old one's average velocity is not more than a percent or so higher on average. This approximation for a rifle keeps you close to SAAMI guidelines for a pressure increase in aging ammo. Of course, you also have lot-to-lot powder variation, which can also make a 1% difference, but this gives you a sense of where things stand. If your velocity changed much you may move off a favorite load sweet spot anyway, in which case you may want to pull the other bullets just to adust their charge weight.