Just happened again a few minutes ago, this was the third time since yesterday. You kill all enemy NPCs, and hostages won't switch to their "normal" state. It's impossible to finish this activity.
I've equipped the scanner pulse and as you can see from the screenshot, there are no enemy NPCs around.
Just in case if it's important, in this last case there was an open world boss near, so the hyena enemies were mixing with the black tusk (from the execution). The second time there was some random small patrol mixed with the execution activity. I wasn't paying attention when it happened for the first time.
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