Update on this: removing an unrelated library (Stetho) lets me build. I wonder if I've hit a bug in Gradle?

And it doesn't really matter which library I remove, it's just a matter of reducing method count. Which is weird, because that's exactly what multidex is for. It's like I've hit a second, unknown method limit.

Finally found a solution: https://stackoverflow.com/questions/47814828/multidex-app-still-over-64k-method-limit