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If it's like the base game, it might be quota based, and is weighted (fairly heavily from the sound of it) towards giving you those classes at the moment.

What exactly do you mean by "quota-based" and "at the moment"?

I'm on the next supply drop now, still getting techs and assaults exclusively, in some cases even 3 of the same class. I really tried this exhaustively and reloaded about 25-30 times on that same drop last night, to no avail. Does this mean I'd have to just wait and see if/when this will eventually go away? Or can I actively do something, wether in the game or by fiddling with some file, to make it stop?
The only thing I have changed about the blackmarket in any config files was to edit he BM-Loot tables from the grimy loot mod I have installed, but that should not influence the classes of recruitables there, right?

Before this started happening (~6 months back in the campaign), I got a fairly even distribution for the most part, though it did tend to give me 1 grenadier or technical in about 3/4 drops (quota?). In 9/10, it gave me at least 1 soldier of the only custom class I am using. I suspect that was because I had less of those in my roster at the time.

But the rest of BM-Recruits was mostly a good mix of all 8 LW2-classes. I did notice on my multiple test-reloads that it seemed to prefer a selection of 3-4 classes on the same drop though, but every once in a while, it would spit out a soldier with a class not from that prefered selection.

From how I understood it, the game tries to supply you with classes that you're either missing or low on, but I'm not sure if Long War changed any of that.

okidoki. thats easy to test. I'll just dismiss a few dudes from the barracks so I get a roster with an even class distribution and try again. But seems weird, I've trained about a dozen assaults over the past few months in-game, they are definitely not the class in short supply on my avenger. Shinobis on the other hand...