So this came up last night, and I ruled Yes, it does spoil it. I let the group know this was a one-time ruling as I wanted to post here to solicit an answer to set it in stone for the table.
It has been previously agreed a Gloom Stalker Ranger in darkness has advantage on every attack against creatures that require darkvision to see:
Does a Gloom Stalker making ranged attacks while in darkness have advantage on every attack?
I think the key point here surrounds this question:
Does pulling / drawing / shooting a Flame Arrows enchanted arrow emit light from the fire? At what point is the arrow or javelin "flaming", if at all? And if it’s prior to impact, does it spoil "darkness"? If it doesn’t flame, how does it cause 1d6 Fire damage?
There is nothing in the RAW for Flame Arrows which states that the arrows are flaming while drawn or in flight, but there is also nothing saying that aren’t. So I think this comes down to DM judgement call. I ruled that, like a Fire Arrow, it’s on fire before launched, and thus, makes the Ranger visible.
What do folks think, and why?