ScriptEngine "js" randomly means either "rhino" or "nashorn", but should instead select one

See the minimized test case attached. Running it, produces the random interleave of Rhino/Nashorn implementations, but I would expect only "nashorn" there:

$ ~/Install/jdk8b83/bin/java ScriptEngineTest
dk.nashorn.api.scripting.NashornScriptEngine@eafc191
jdk.nashorn.api.scripting.NashornScriptEngine@52e677af
jdk.nashorn.api.scripting.NashornScriptEngine@61df66b6
jdk.nashorn.api.scripting.NashornScriptEngine@6913c1fb
com.sun.script.javascript.RhinoScriptEngine@5aebe890
jdk.nashorn.api.scripting.NashornScriptEngine@5ba3f27a
com.sun.script.javascript.RhinoScriptEngine@6fd83fc1
jdk.nashorn.api.scripting.NashornScriptEngine@57bc27f5
jdk.nashorn.api.scripting.NashornScriptEngine@2fba3fc4
com.sun.script.javascript.RhinoScriptEngine@a37aefe
...