Thibaultmol Found out that myself too. They font is correct in private window mode. I probably have the wrong Helvetica font, I've deleted all of them. 😛
I'm guessing they default the font on Kagi to Helvetica first because Apples OS' come with the actual Helvetica font, and Windows doesn't.
Activated custom CSS and added
body { font-family: Arial, sans-serif !important; }
Problem fixed (lol)