It's not new (e.g. see https://forums.d3go.com/discussion/comment/888973). Matching 5 twice in the same cascade still gives only one extra turn most of the time; it seems to give two when one of the match 5s involves a permanent support on the board. May or may not be a bug, but it happens so rarely it's mostly just an interesting easter egg whenever it does.
The behaviour seems pretty consistent, actually - but it's certainly obscure and unexplained.