Sorry, I got a bit confused: The different behavior for routines index and rindex with negative startposition between rakudo-moar and rakudo-jvm was fixed (see https://rt.perl.org/Ticket/Display.html?id=126700).
What's still relevant is the different implementation of nqp::index:
$ perl6-m -e 'use nqp; say nqp::index("xxy","y",-1)'
-1
$ perl6-j -e 'use nqp; say nqp::index("xxy","y",-1)'
2
Output generated with Rakudo version 2016.03-110-g5bfc8fb.
