Not sure if it's a bug or not, but characters with lower skills, say roughly mining level 8 and get tired after about 3 to 5 tries (guessing based on discussions with other players) initially don't get much gold but after they work at it for a while, especially if they get themselves all tired out a couple times, seem to mine gold very well, maybe 1 in 2 tries (I think that is an over estimate but that is what I was told).
Now consider that a character with mining level 20 and all stats at 200 does not get tired out at all when mining and it seems as a side effect that character does awful at mining, say 2 in 30 tries. I'm not saying that happens to all characters with high stats but that is what I'm seeing.
In other words it seems that two "features" may be confliciting. I'm guessing that the design is that you have to work at gold mining for a period of time or effort before you can get gold more than say 1 in 50. Let's say that effort is measured by how much the character gets tired out.
Second, let's guess that with high stats and high mining skill the character does not get tired out, result catch-22 that character should not bother mining gold (or silver it seems).
I've mined right next to the player I was talking to about this and I did awful and he did great for a long period of time.