I do know that some bots don't work very well in Robocode 1.9.2.x and/or Java 8. See f.e. Xiongan.xiongan and tcf.Drifter, they get scores of 0 against GrubbmThree, while on my system (126.96.36.199 and Java 7) they just work ok.
There is else but Java 7 vs 8. I ran tcf.Drifter vs. GrubbmThree 0.9d in robocode GUI with Java 8. Drifter constantly wins the match.
So I cannot attribute 100% loss which we see in rumble just to the Java version.