If you perform an async operation you need to subscribe to it, otherwise it will not be executed.
For blocking, try .upsert(document).toBlocking().single(); or if you want to still be async use .subscribe() and variants, but keep in mind that this won’t be executed on the current thread so don’t use .subscribe() and then let the main thread exit.
The documentation has lots of stuff on this topic!
The reason why it did work on 2.1 is that the observables were “hot” back then, but your code was still wrong from a contract perspective - you always need to subscribe to an observable to execute it.