I don't know what caused it, but I was shooting arrows while looking for a glitched/invisible horse (which are all over in my pasture). I exited Minecraft and came back and the arrows were like that. Three or four times it happened. Vanilla 1.12.2.
Of course, I know the arrows came from me, but this is a large biome world and I've not even found a frozen biome at all, so there are no Strays here. I've never collected dragon's breath and these were regular arrows.