ThreadDeath problem
Well, println() is definitely broken somehow. There is no way that checkSkippedTurn() could have called forceStopThread() (as is shown in the stacktrace), without going through println(), yet the print message never made its way to the console.
battleText is also cleared in initializeRound() and [https://github.com/robo-code/robocode/blob/0eb93ba532bc6731b7ff8dd02da1308d92ccb2fd/robocode.battle/src/main/java/net/sf/robocode/battle/peer/RobotPeer.java#L1685 cleanup(), both of which have no synchronization guards at all.
Good catch. I've added synchronization guards on the initializeRound() and cleanup() and now I get the SYSTEM: Neuromancer 5.4 has not performed any actions in a reasonable amount of time. printed inside the correct round. The ThreadDeath is still printed in the round after, but I can live with that I guess.
Now for the next question, why is this even happening?
It seems that Skilgannon has filed a bug report on this issue, including the necessary patch.
To add to mysteries. Why I do not see dramatic drop in scores if I run Voidious' RoboRunner? I see missing SYSTEM messages in logs but it does not affect scores the same drastic way as with the literumble client. I also see no ThreadDeath messages. I am guessing RoboRunner catches those somehow and process the score differently.
