When logged into the site, the header isn’t replicated, but it is when live. Yes, one would assume it’s a caching issue. However, if you make an update elsewhere on the site and publish it, clear the cache, etc. that update is shown on the live site, but the header is still replicated. That would indicate that there is still an issue causing the header to replicate on the live site, when not logged into WP. The only work around I have is to delete the header and instead insert an image to act as a header. It’s not ideal but it’s the only work around since your team doesn’t believe there’s an issue.