Well, in fact this was already discussed in
MNT-1415 , there was stated that it won't be fixed but no reasoning is present there.
endStartTask parameter is parsed like this:
// determine whether to auto-end the start task
Boolean endStartTask = (Boolean)ruleAction.getParameterValue(PARAM_END_START_TASK);
String startTaskTransition = (String)ruleAction.getParameterValue(PARAM_START_TASK_TRANSITION);
endStartTask = (endStartTask == null) ? true : false;
It "seems" quite obvious that the last sentence should be:
endStartTask = (endStartTask == null) ? true : endStartTask;
I can imagine that this wasn't fixed due to backward compatibility issues but this change would make the default value still be the same and at the same time honour the right parameter value not only when it is false but also when it is true.
It doesn't seem very reasonable to expect that people who used the action and set the parameter value explicitly to "true" would expect it to behave like false, so in this case they most probably did not use the parameter at all, which won't break.