Perfect on Desktop, not working on mobile main page

Set up ‘header’ as sticky element. z-index is 999999 (zero, allows elements to overlay the header). This works perfectly on desktops with all pages, but on Android Chrome (latest version) the main-page header doesn’t stick and the “reference library” page drops the header down a ways from top of viewport screen (it looked like that when I was fiddling with “z-index” setting at 100. All other pages work perfectly on mobile, too. I have cleared caches, gone incognito, etc. Why the unique behavior on different pages using Android chrome? Legacy off.

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