So in theory this should be resolved in https://github.com/jMonkeyEngine/sdk/releases/tag/v3.2.1-stable-sdk2
If you want you may try this again with java 9 (what it does is it comes with it’s own jdk8).
I tried it on my mac but since I’ve had java 8, I did not have the issue. I can’t tell you if it’s fixed because the installer still prints “Looking for System JRE”, so the bundled one seems to be only a fallback in that case.
Anyway, if someone is seeing this thread and having issues, feel free to notify me.