Sorry if this has been already posted before, but I've noticed a problem when talking with some npcs, that certain dialogue topics that I have already selected in previous coversations with a given npc revert back to the bold, white font instead of staying darkened out like they should. An example is the housecarls (asking "what does a housecarl do?" and "what is a thane?"). Other examples are Ysolda in Whiterun (asking "what can you tell me about sleeping tree sap") and Phinis Gestor at the College of Winterhold ("You mentioned missing apprentices"). It also seems to be extremely common with npcs in Solsteim with the Dragonborn DLC. Does anyone knows what triggers this issue if anything? I hope my description of the issue isn't too confusing.

I have also noticed this, and from my own testing, the reason for its occurrence seems to be based on two things; (In my following descriptions, I use the term "initial dialogue option" as the top-most option that you select, and "subsequent dialogue" as a term for any dialogue that follows it).

1) After selecting an initial dialogue option, and (after seeing the subsequent dialogue) the initial dialogue option continues to be in white bold (and not grey) is because there is extra subsequent dialogue (in the form of a menu) that hasn't been seen yet. Until all subsequent dialogue relating to the initial dialogue option has been seen, the initial dialogue option will remain white bold. This most frequently happens when the initial dialogue option has multiple subsequent dialogue options available.
2) The above (i.e. the initial dialogue option remains bold) can also happen when there is only one subsequent dialogue option. However, this will happen because there are multiple 'versions' of the subsequent dialogue (the dialogue in some cases is almost identical, but there will be differences, as if an NPC says the same thing but in a slightly different way, perhaps in the form of a summary).

The two points above are not bugs, but are simply ways of showing the player that there are 'unseen' dialogue options available. When all dialogue options (or 'versions' of dialogue) have been seen, then the initial dialogue option will turn from white to grey.

However, the following point 'is' a bug;

3) After reloading a savegame, many of the initial dialogue options for an NPC (for example, a follower) will turn white again, even though you know that you have already seen all dialogue options previously. This seems to be a problem that either, a) the savegame is not registering and recording the fact that you have seen the dialogue options for an NPC, or after reloading, the dialogue options were saved but the dialogue menu is not being setup correctly when you next talk to an NPC. I have noticed that after reloading, certain dialogue options turned white again, but after a while, they turned grey as if the game suddenly remembered that you have already seen those options (I 'think' it happens when you change cell, i.e. fast travel or do through a door, but it doesn't always correct itself).

Thanks for the information, it occurred frequently on my first playthrough and just now started on my second playthrough. On my second (current) playthrough that I started after the 1.8 patch came out, the issue started around the time I hit level 45 and my save reached about 100 (real-time) hours, although neither of these things probably has anything to do with the bug. I think I started noticing this on my first playthrough after downloading the 1.6 patch update from xbox live.

Yes, you are right in thinking it is patch related. I have been playing Skyrim since it came out last year, and for the first few months, this bug never occurred (i.e. with reference to point 3, where certain NPC dialogue options turn 'white' after reloading). However, after one of the patches (it could well have been patch 1.6) that is when I noticed dialogue options turning white after reloading, and seems to have been in the game ever since.