The bug

The height generation of iron and coal ores jumps sharply at certain Y levels, due to overlapping of flat and ramped distributions. For coal it is around Y=136 and for iron it is a smaller jump at around Y=-8.

This is happening for coal and iron because a flat ore distribution overlaps with the tail of a triangular distribution but the flat distribution does not extend over the whole of the triangular distribution. Compare this to the likely correct behaviour of redstone ore above Y=-64 where the flat distribution does not overlap in this way above Y=-64. However, redstone ore may also be affected below Y=-64 if the flat distribution ends before the triangular distribution (I have not tested this because it requires a custom world).

This is a mathematical problem in the code and it can be fixed with a mathematical solution. Two fixes are possible: (1) Create a "ramp" distribution that behaves like a triangular distribution but only extending in one direction. The ramp can be combined with a flat distribution to create ramps. (2) Amend the triangular distribution to include a maximum value.

How these fixes would look with a simplified numeric progression. (1) 0, 1, 2, 3, 4, 5, 0. (2, clamped at 4) 0, 1, 2, 3, 4, 4, 4, 4, 4, 3, 2, 1, 0.