pt-online-schema-change stopped while running on a 1.5 million record table

Thank you so much for the clarification. In fact we did further investigation to understand why it stopped abruptly and we figured out that the issue.
Before the end of the day, we logged of the Linux machine where the tool was running and the tool script has a function to handle signals and exit on SIGNUP

We commented those line and successfully ran the entire alter. It took two days to complete!