It's not uncommon to get real poor performance from factory loads, or poorly constructed handloads - especially when dealing with big pellets in smaller bores or, in this case, buckshot in a muzzleloader.
Have to touch on nitro cartridge loads just for a point of reference. Typical factory buck loads are tailored for self defense to be used at ≤10yds (30' which is about the maximum across-the-house shot), those marketed for hunting use are tailored to use at ≤20yds and no one likes to show-off ugly patterns so all anyone typically sees are the cherry-picked pattern pictures. Of course there's all kinds of marketing sales hype involved but when I put some of the alleged "premium, MAGNUM, premier, gold" bla bla bla ... ammo to paper, the results weren't anywhere near as impressive as the sales hype.
In most cases one could expect patterns of 16-24" at 25yds with just about any generic 12ga load except for 000B size pellets where 3 of the 10 factory loaded rounds tested in two different guns failed to put a single pellet inside the 20" target ring. I also tried a variety of 27-pellet loads of #4B, typically one could reasonably expect to see about 50% of the pellets inside 20" @ 25yds ... but ... it's also pretty much guaranteed that the pattern will be blotchy with clumps of pellets here and there with big open holes between them.
I tested five brands of 20ga with 20-pellet payloads of #3B in three guns, all patterns were erratic, there wasn't any real consistency at all. One would throw a 2-3 blotches of 3-4 pellets, next round may throw a decent blotch close to the POA while the next could be a widely dispersed pattern.
A few things are consistent no matter if throwing buck from a cartridge or a muzzleloader:
1- A pattern at 10yds is not going to be indicative of the pattern at any range beyond the 30' line. A tuned load can print good consistent patterns at 25yds and by 30yds it's fallen apart to the point of being worthless.
2- Pellet stacks in a 2x2 arrangement rarely throw consistent patterns.
3- Pellet stacks in 3x3 arrangement lacking a sufficient amount of buffer material tend to produce more fliers and blotches.
4- If you let the pellets ride against the bore (not using a cup/sleeve), you may as well not even bother loading the gun.
5- If you can't keep 100% of 00B or 000B pellets inside a 16" circle centered on the POA, the load is inconsistent and unreliable.
6- If you can't keep 75% of #3B or #4B pellets inside a 20" circle centered on the POA, the load is inconsistent and unreliable.
7- A higher pellet count does not necessarily improve pattern density or consistency.
8- The harder and rounder the pellets, the better they'll pattern - commercial buckshot, even Ni or Cu plated, is not hard - the plating only helps reduce some velocity loss from drag after impact.
9- The total payload of the entire load must be accounted for (wads, buffer, sleeve/cup and pellets).
10- The slower the load accelerates, "tipically" the less problematic it will tend be.
I recall seeing a quote on another forum concerning 00B loads in a 12ga cartridge gun:
I want a load I feel confident with if I run into a 400lb bear at 40yds.
After pushing a lot of buckshot downrange, I can assure you there isn't a single load I would feel anywhere near "confident" with on a 150# bear at 40yds let alone a 400 pounder ... perhaps a couple pounds of .54 balls loaded as buckshot in a 155mm Howitzer but I have no confidence in anything that's shoulder-fired to throw buckshot at long range.