According to this article, the tiebreaker rule was introduced in 2014, but not officially announced until 2016, and the first tiebreaker didn't happen until 2018:
https://www.theringer.com/tv/2021/1/...-rules-changes
When they do happen, it's usually because two players were tied at the end of Double Jeopardy and both bet it all and were right. Last night's was a rare tie caused by what I assume was a math error on the part of the leader going into Final.