I just want to point out that playing Devil's Advocate is not as innocent as you make it seem. Most people, when they say they're doing it are posting poorly supported populist arguments, based on some unsubstantiated folk wisdom or straight up fake facts. Not saying you did that in that particular case.
Playing Devil's Advocate is only valid when you're dealing with normative scenarios, for example when advocating for one policy over another, both of which have drawbacks and advantages.
When someone says they're playing Devil's Advocate for issues where the science has been mostly settled, it's usually what I stated above in the first paragraph, and becomes very clear that the poster has some ulterior motive or is personally attached to their beliefs over evidence.
So the reason those get downvoted is that when you have a wrong populist argument at the top of the thread, more people see it and adopt it as their position, thus perpetuating the cycle. To break the cycle, people downvote it.