Yes, the code in the first post was copied directly from the source in the two browsers. The full set of nav links are completely missing. I haven't been able to track down any JS or browser plugins which could have removed the HTML elements. I disabled all JS, ad-blockers etc. to see if it made any difference.
Really stumped by this. I can't believe it could be PHP altering the output. Think it's much more likely a browser and/or user error, but I just can't track the damn thing down.
Anyway, I've hard-coded them for now, and luckily the nav pages are unlikely to change. I'll just have to circle back to it once the project's finished for my own sanity!