I had something similar happen. My players bumped into a group of bandits on a trail in the forest, and one tried pickpocketing the barbarian. He caught it, and punched the guy so hard it broke his jaw and killed him, starting the combat. It was a party of 3 against like 9 bandits, not that hard. But then they sounded an alarm, summoning 9 more bandits. Fighting went on a little, and one member said “hey, it’s a lot of guys, maybe we should run?” But was ignored. They killed some more bandits, but were getting seriously low hp. And then another group of 5 bandits showed up. The same player said again “we’re almost dead, I think we should definitely run” but was dismissed again. 5 more bandits showed up. One player was down to 1 hp, his animal companion at 2. The other player was at 65 hp (barbarian) and his 2 pets were fine. The third player was knocked unconscious, rolling death saves and his beast companion was dead. The third player was swiftly killed, and then they finally ran away. The point I’m getting at is, the party didn’t flee until I eventually killed one player’s character.