There are many conversations between NPCs, which can be found in the creation kit, that I have never heard in hundreds of hours of gameplay, and on three separate characters. If they play at all, it must be ridiculously rare.
I remember a similar issue with scenes in Skyrim. Apparently it was sometimes caused by their quests being "configured as Actor Dialogue Events but had not been properly configured to start the scenes they were meant to call."
An example of these scenes is: ConvBoSAirportPrydwen01Scene ---- through ---- ConvBoSAirportPrydwen08Scene
like the Skyrim bug above, these seem to be Actor Dialogue Events.
My example is from the Prydwen, but there seem to be bugged scenes in a bunch of places, like Railroad HQ.