I just started having this same problem in the last couple of months. It's a gray triangle with an exclamation point that shows up by 2 of my contacts in the Conversations list in Messages+. Both contacts have iPhones and mine is a Galaxy S6. Clicking on the triangle just takes me to that thread. No idea when it first showed up or if it pertains to incoming or outgoing messages. Do you have any info on this since the first question was posted in 2017?
I would find that symbol annoying at best myself. Let's get a few details. That symbol usually means there is a message that was not sent. Usually, this message would be towards the top or even out of order. Have you scrolled through the thread to see if there were any messages that failed since you notice this symbol?
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