strange CSS behavior on posts page : wrong color menu is applied

Hello,

Since few days, i've noticed that my main menu have a strange bug on posts pages (only). Normal menu and sticky menu are both working normally on pages. Sticky menu is working normaly on post pages (after it displays on scroll), but normal menu inherit a wrong white color from a css declaration, and i can't find it in my child theme. ofc i desactivated all my plugin and cache to try to find out...

It's weird, because the menu works perfectly everywhere, but not on posts page only. Even more strange : the bug occurres only on production, i can't reproduce it locally :s.

+ i have a second child theme for other subsites of the network, wich is a clone of the childthme i'm using, and eveyrhting is normal... the menu is shown normally on post page.

You can check the example page linked in the hidden field down there, and just click on the logo to display the main page and navigate through other pages..

Help appreciated if you have an idea, i've already spent few hours on it, never see this behavior before.