Unfortunately, the 'goodbye' option seems to be missing from certain conversations with some NPCs. I don't remember this being an issue from my days playing the console version which leads me to believe the issue has been caused by a mod or two. Does anyone know of a way to fix this? Or maybe if there's a mod which makes the dialogue options similar to Skyrim so that you can just walk away mid conversation? Any help would be appreciated!

What's your load order?

its one of you mods do you have any compaion mods or npc recruiting ones they can conflict with each other so disable your mods into you find the culprit

Its much easier,open up your console and type "Cam" or closeallmenus.That should fix any in-game menu wich does not want to close.

Generally speaking, this is caused by an issue with the GOODBYE topic under Conversations. Most likely, something you've modded, or installed, is causing issues with that.

First, Save near a NPC who has this problem

Disable your mods, then enable them one by one, always checking in between whether or not the problem persists. Once the problem reappears, you know you've activated the problem mod.