SOLUTION (for me)
For the chat it was happening in (RCP texting with three iOS phones and one Android) I was suddenly getting Haptic notifications despite having the chat muted. I tried several things, including turning on and off the Hide setting on the chat and the individuals. This sometimes worked for a bit.
What solved it for me was going into each individual contact (I went through the "5 people" menu at the top of the chat record), selecting "Edit" and setting each person's Text Tone sound and Haptic vibration to "none."
That makes some intuitive sense but should obviously (as in the past) not override the selection you make for the chat as a whole. It also might create problems if you want to mute the chat but not the individuals, but hey.