Unpredictable js caching issue

I have an unpredictable caching issue which is hard to track down.

I have a sticky menu header with a white background which shows when I scroll the page. When the page first loads the menu background is transparent.This is the correct behaviour. It never fails when in development mode on cloudflare or when logged in. What seems to happen is some time later, perhaps hours or even days, the page (not logged in and cached) no longer shows the transparent menu background but instead it behaves like it has already scrolled down to give a white background. I can roll up or down but still the white background remains fixed. The javascript which inserts a transparent menu is not firing.

WordPress 7.1.2 WP Bakery 9.0.1 Super page cache 5.3.5