[Fizzlemizz]

watchedFactionID = watchedFactionID == nil and inInstance and hasDungeonTabard and tabardID

That resoves to a boolean

if watchedFactionID == nil and inInstance == AnythingButNilOrFalse and hasDungeonTabard == AnythingButNilOrFalse and tabardID == AnythingButNilOrFalse then
   watchedFactionID = true
else
   watchedFactionID = false
end

which means watchedFactionID won't be nil for the others because it will always be true or false (boolean).


Maybe:

watchedFactionID = (watchedFactionID == nil and inInstance and hasDungeonTabard) and tabardID or nil

if watchedFactionID = (watchedFactionID == nil and inInstance == AnythingButNilOrFalse and hasDungeonTabard == AnythingButNilOrFalse) then 
    watchedFactionID = tabardID
else
    watchedFactionID = nil
end

(Define your truth) and (then make an assigment if true) or (other assigment [defaults to false if no "or" and truth is not met])