I assure you no one doubted you. We try to reproduce an issue in order to better understand the issue itself and how widespread it is. Not to discredit you.
When you send a long text message, by the nature of SMS, it is broken into multiple, individual SMS messages. Because you are sending all as one long message, all these individual messages have the same timestamp. Since the receiving iPhone can apparently handle only messages up to the length limit @drm186 cited, and they all have the same timestamp, they appear on the phone out of order.
Edited to add:
I asked earlier whether you get the same result when you text from the phone. Could you let me know the answer?
Once I have that answer, I will show our app developers what you are experiencing and ask if there is anything they can add to the metadata that might help the receiving phone better order the messages.