Postscript: I had assumed this was a worn plate. However it doesn't fit because it occurs only in this colour over a long time period. If it was a worn plate it should only occur in one time frame before being repaired or replaced. Then I read an article by Gordon Monk et al on compartment line research relating to the Australia 1d red George V issue. Here is a relevant extract:
Surface printing is notorious for a problem called ‘ink build up." Printer's ink is basically a bonder such as linseed oil. Pigments (usually in powder form), additives, such as “brighteners” and solvents. It is the solvents that are the main culprit for the problem of ink build up.
When the plate is inked again a new layer of ink is applied over the previous deposit that is again not picked up by the paper. As the solvents evaporate, layer after layer of ink is deposited on the edges of the printing surface and the end result is loss of definition. The spaces between the shading lines become smaller up to the point that they fuse together. It takes many applications of ink for the loss of detail to become noticeable; however, when it does the process seems to accelerate.
And this is what appears to have happened. I would suggest that the reason it only occurs with this particular colour shade is due to the specific combination of pigments that make up this particular colour shade.
Here is an extreme example
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