How to fix iTunes backup missing some SMS ?

My paid app (which relies on iTunes non-encrypted backup before it can export to CSV) claims some of my missing SMS export is due to their non-text content.

However, there’s two reasons why I suspect iTune backup is the problem.

① website says SMS has no non-text content https://blog.textedly.com/difference-between-texting-and-instant-messaging),

② this App has claimed one of my previous iTunes backup is corrupted.

③ missed SMS export is no different from successful exported SMS except dates and amount.

I need help how to

① differentiate non-text content of SMS when viewed by iPhone Message.

② know if iTunes hasn’t backup all my SMS messages?