I've been having the same issue recently, and it always happens with my husband's texts. The phone is at full volume, sitting right next to me on the arm of the couch, and I'll see that a text came in, but the sound never played. It's frustrating.
I have an 8+ and have noticed it as well. BUT, I also had this issue with my 6s and my wife has the same issue with her SE. I've observed this issue since from iOS 11 at least. Very frustrating.