https://www.theverge.com/2016/1/17/10777102/upside-down-smile-emoji-sarcasm-ambiguity

The upside-down face emoji is headed for trouble. Approved as part of Unicode 8.0, it rolled out to iOS 9.1 and Android Marshmallow earlier this year, and in the past three months, the meaning has become remarkably consistent. Dropped at the end of a sentence, the face typically suggests the previous statement should be taken in jest, like so: