Paint on doors and windows changes from brown to purple

We renewed wood windows in our house between 1971 and 1983. Every few years they were treated with a new coat of paint or glaze.
The last coat of paint in 2014 was with a high-quality window paint RAL 8016, as stated on the invoice, but after just 2 years its color already changed to purple, more or less strongly in direct sunlight.
What makes me suspicious is the fact that through coatings between the original state 1971 and 1983, a full 43 or 31 years, no color change occurred, at most some fading.
To avoid making mistakes again, I would like to know what the fault is, wrong paint??

The picture shows a door, southeast position, with different colors, covered and uncovered.