It is interesting to note that mobile view (or decreasing browser width) causes the background to show up. It only appears to be gone in larger views.
Followed your steps, to no avail:
- Everything is up to date
- All caches disabled & browser cache cleared. Tried multiple browser types, computers & ISPs.
- No CDN in use. I'm only using Cloudflare for DNS.
- Disabled all plugins.
- Deactivated child theme & switched to parent theme. Removed all global custom CSS (which I believe is all that could be affecting that particular page).
- Increased memory allocation in wp-config.php.
Tested after each step and issue persists each time. I will reply privately with logon credentials if you would like to be able to log into investigate. The site is currently in development, so you can feel free to disable plugins as necessary.
Another note: have another site on Pro in which this is working just fine ... Thus, there is obviously a conflict somewhere, but not sure where else to look.