Take it out and thoroughly inspect the tail light.
^^THIS^^
I've only ever heard of this issue (on any car) where the light was cracked somewhere, which obviously required a repair or replacement, so l would look into it further....But if your lights have gaskets or some kind of seal around where the bulbs mount into the fixture, that may also be the problem, which may or may not apply to an '18, but is worth looking at just to be sure
I know the LED replacement bulbs I put in my '011 have rubber gaskets on them, which not only seals out any water/condensation, but also ensures a very tight, secure fit too....