Non-iMessage SMS texts hidden and inaccessible in conversation, but accounted for in unread messages badge

I recently traveled abroad with my iPhone 6, and sent and received text messages to another iPhone user while on International Roaming. Since I did not have internet access, the texts were transmitted on SMS (green) and not iMessaging (blue). On my return to the States, all of the SMS text messages disappeared from the conversation. The Messages app, however, now has an unread messages badge with the number corresponding to the missing SMS text messages.

The only way I can access those messages is if I ask Siri to "read my unread messages" at which point Siri does in fact read the hidden messages, but those messages are still hidden from the conversation, and the messages are still considered unread (the unread counter does not reset to zero).

How can I make the non-iMessage SMS texts become visible in the conversation?