When reactor timer tasks are scheduled with the same deadline, there is some handling in the task compareTo handling which looks to impose a first-scheduled order on them. This isn't effective though as the counter value used in the effort is erroneously always equal. Each timer should index its tasks to ensure they can be ordered on the expected first-scheduled basis when tasks have equal deadlines.
Looking at the implementation, it seems that every instance of TaskImpl has this.counter == 0. This seems wrong. The count field should be static for the instance id to work.