They made the contact photos option apply to the conversations themselves (at the top) as they apply to the conversation list.It also showed up when I connected my phone to my bluetooth speaker to listen to spotify. I get it Apple, you don't have a headphone jack anymore but not everything connected to the iphone via bluetooth are headphones. I'm using a 6s by the way.
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I noticed this as well on my 6s. All my group texts show the total user count like you said at the top of the text. I did find if you toggle "Show Contact Photos" on in the Messages settings, it will then go back to showing the people as before instead of just the total.
Of course you have to see the photos in the list of messages on the side now, which I had turned off for a reason. Thanks Apple for making a change for apparently no good reason...