Iphone deleting sent iMessages in a conversation but not normal texts?

I started texting someone yesterday and at first we were using normal texts, but then realised we both had iPhones and used iMessage. I texted them this afternoon but didn't look at the previous messages. They didn't reply which I thought was strange and so I went onto the messages to see etc and while doing so I realised all the iMessages weren't they but the normal ones were so I don't know whether they've replied? Does anyone know what this happened?

It's probably in a separate conversation (like a different thread) because you probably sent the iMessage to their Apple ID (email address) instead of their phone number. If you want to have iMessages and Text Messages in the same thread then make sure you send iMessages with your number to their number.

You can also try

Settings < Messages < Send as SMS < OFF