Error with getting a message 2 days later

A few days ago I sent a text to a friend, it said delivered. The friend has read receipts on. I didn’t receive a response and it did not say that the message was read.

2 days later my phone lagged a bit. Suddenly there was a notification that I had a message. I scrolled down and saw my friend’s response, it was from two days prior. It had not showed up as read or even on my phone the days prior. I updated my phone to the most recent iOS, so I am hoping this issue won’t happen again.

Any ideas on what could’ve caused this?