This does look like classic pushing the metal too hard through the drawplate (or rolling mill).
A friend of mine told me one of the best explanations of why this happens. It’s two-fold.
First, cast silver, gold, etc is kind of like hyper-annealed metal. The grain structure is spread so far apart that the metal is actually brittle. With a cast ingot to bring it to a malleable state, you have to do what Rob says. File and sand off any parting lines or imperfections. Then forge, anneal and pickle at least 5 times before touching the rolling mill or drawplate. (Some people say to forge, anneal, pickle enough to reduce the thickness by 20%. I’ve found that approximately 5 times works for me.) The idea is to compress the grain structure and strengthen the metal, so it can withstand the stress of rolling or a drawplate.
Second, my friend said to think about the metal like an ocean wave where the water on the surface moves at a different rate then below the surface. That’s exactly what happens with milling out an ingot. The outside of the metal moves at a different rate than the inside of the ingot.
That’s what it looks like here. The outside was moving more than the inside which created stress fractures on the surface.
In addition to doing the forging, annealing pickling that Rob suggested, you also probably need to anneal more frequently when drawing or rolling, especially at first.
Sure, there’s other possibilities, especially since you said there’s solder in your mix, which probably is a mixture of silver, copper and zinc. Zinc can cause all kinds of unexpected problems sometimes.
Personally, I don’t see any problems with your gases or torch set up. If it’s melting the metal, it’s melting the metal.
This is hard to diagnose from a picture, but that’s where I’d start.
Hope that helps!! Maybe others have some different useful tips.
Jeff