Dear Ben,
welcome to the forum, and thank you for reporting your problem.
May I ask you to append the RETURN SUMMARY
clause to your COPY FROM
statement? It might give you a clue already about what is going wrong.
For more information about that topic, see also the excellent Fundamentals of the COPY FROM statement document by @Marija.
This could actually be the cause of the problem:
With kind regards,
Andreas.