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There are several possible reasons why the Java merge sort algorithm may print unpredictable results and fail to sort the ArrayList, including:

1. Incorrect implementation: The code for the merge sort algorithm may contain errors or mistakes that prevent it from working correctly. This could be due to syntax errors, logical errors, or other mistakes made during coding.

2. Incorrect input data: The ArrayList being sorted may contain unexpected or invalid data that the algorithm is not properly equipped to handle. For example, if the ArrayList contains elements that are not comparable or orderable, the algorithm may fail to produce a sorted output.

3. Memory issues: The merge sort algorithm requires a significant amount of memory to work properly, and if there is not enough available memory or if there are memory leaks or other issues in the code, the algorithm may behave unpredictably or fail.

4. Concurrency issues: If the merge sort algorithm is used in a multi-threaded or concurrent environment, there may be race conditions or other synchronization issues that can cause unexpected behavior or failure.

Overall, the Java merge sort algorithm requires careful implementation and testing in order to work correctly, and any issues with the input data, memory, or concurrency must be carefully addressed in order to ensure reliable and predictable behavior.