You put “use British English spelling” in the system prompt. The first three paragraphs are fine. By paragraph nine there is a color, and by the end there is an organization. The instruction was not ignored; it was outvoted.
The symptom
The characteristic pattern is not uniform failure. It is a document that starts correct and degrades — and the degradation is usually inconsistent within the document, so you get colour in one paragraph and color two paragraphs later, sometimes in the same sentence as behaviour. Long outputs are worse than short ones, and a long conversation is worse than a single call.
A second symptom is domain-specific: the spelling holds in ordinary prose and fails in technical contexts. Code comments, API field names, CSS properties and library names are American by convention (color is a CSS property; serialize is what the method is called), and text near them pulls the surrounding prose across.
Both patterns point at the same cause, and it is not that the model did not read the instruction.
