Now, the bug itself apparently isn't uncommon. After getting The Grand Staircase and a bunch of other quests from Daniel in the Honest Hearts DLC, right after talking to Waking Cloud, Graham spawns like ten feet away and I'm only able to initiate the same dialogue I could have with him back at the cave. I found a few more instances of people encountering this bug, the issue itself seems pretty by the numbers with the solution being a straightforward copy-override, the issue is if I open FNVedit there are literally no mods in conflict with any of the relevant dialogue or scripting and only YUP even touches them. If it's relevant the first time I encountered Joshua at the cave I got "NVDLC02JoshuaAboutZion" listed as a dialogue option, which I thought was weird but minor.
I'm sure if I shared my mod list it would give veteran modders a heart attack because it's over 300 (I know, bad idea, but I'm a glutton for punishment and I saw making way too many mods work together as an interesting challenge, believe it or not I've not seen that many bugs other than the sporadic crash and some clipping issues) but I swear to Christ FNVedit isn't showing any of them impacting this dialogue nor NVDLC02JoshuaNarrowsForceGreetTriggerSCRIPT besides YUP. I did try copy-overrides of both those things just to be sure, but it changed nothing. Burning Campfire 2 and Jsawyerultimate also touch some things in the DLC but nothing related to this bug. I thought for sure Zion Trail/Zion Trail Enhanced would be the culprit for obvious reasons but I'm seeing absolutely no conflicts there.