add_header X-Frame-Options DENY invalidates rt-Fastcgi-Cache?

I added the following to my nginx site conf file:
add_header X-Frame-Options DENY

What I noticed is that if I view the http headers of my site the rt-Fastcgi-Cache parameter no longer shows up. I see nothing about that at all.

If I remove the add_header X-Frame-Options DENY from my conf file, then I am able to see the rt-Fastcgi-Cache in my header.

Why would this happen, are they not totally unrelated functions?

With this problem, is there a better way to make sure that my site cannot be Iframed?

There shouldn’t be any conflict about 2 headers.

For preventing site being used in iframe - you can try this - . Check javascript-based solution they have provided.