Thanks for the reply, sorry it took so long for me to respond.
I finally realized that Netlify takes control of specifically the the x-robots-tag header and always sets it for deploy previews and always clears it for main deploys. In my basic experimentation, I found the user cannot modify that behavior.
In light of this, I wrote some code to add the <meta name="robots" content="noindex"> html tag where necessary which accomplishes the same thing.
Cheers!