You've discovered one of the ugly dark corners of
Struts. I've had this issue as well. It doesn't happen with checkboxes but with textfields - wooo doggies! I believe it would be worth posting to the user list, which can be found on the Struts official site. Since the people there wrote the code, they may be more intimate with how to get around it.
In the meantime, I will let you know my fix. I store a value in session that represents the largest number the index can be. Then in the reset method I initialize the array to the length of this max value. Don't forget to change the max value if you ever have list size greater than the current max value.
Part of the necessity for the max value is that a user can use browser back buttons to submit old data that might have too many indexed values - and Struts will garbage up
before you ever get a chance in your Action to handle the fact that it is an invalid submission.
Best of luck!