Iphone only showing latest sms from one sender

My service provider (giffgaff in the UK) sends an SMS every time I make a chargeable call, to let me know the new credit level.

I see every message come in. However, in that message thread, only the latest message is shown. Notifications still shows the notification for each message, so the phone is definitely receiving the messages - it just seems to replace each message with a new one, rather than listing them all.

This does not happen with other senders (eg the phone shows all my banks daily mini statements).

Is there a setting somewhere to fix this?

Thanks