Need help solving this very weird purity issue

[tongshadow]

I'm having serious purity issues on this set and I wanna know if any of you, which seem to be very experienced in the matter, have encountered something similar. Compared to most common purity issues it's somewhat subtle but noticeable in person. What happens is that the area around the center of the display is having purity issues, as it can be seen in this picture:

Instead of a solid magenta color, Im setting blueish tones around. Same for a white screen but it's greenish/yellowish. Here's a list of things I've tried:

    Moving the yoke forward/backwards
    Replacing the yoke with an identical one
    Adjusting the purity rings only makes overall purity worse
    Degaussing wand
    Can confirm the degaussing circuit is working
    Removing the purity magnets dont affect the screen at all

Im pretty experienced when it comes to yoke adjustments but this is the first time I'm seeing something like this, it's like something around the tube is magnetized. The set is rather old (20+ years) and very used, but otherwise the picture is nice despite the purity issue.​​​​