Question About Lemmy API Versions

I'm working on a little tool for Lemmy using the rust backend (lemmy_api_common crate 0.17.4).

I've tested it against some 0.17.4 instances with no issues, but yesterday I went to test it more thoroughly on the official testing instances and had JSON parsing issues. I realized later that these servers are currently running 0.18.X.

Is this a bug? Or are we expecting that even within the "v3" APIs that you'll have breaking changes between Lemmy versions?