I have seen this before too, also playing 13th Age. Was this a case of one person sacrificing their toon so the rest could keep the ingredients? Usually I keep the campaign loss details to myself until -after- they have fled (which isn't really often). Because there are no clumsy mechanics for retreating, it should be simpler, but...To clarify we werent actually playing 5e. We were in 13th age in which the flight mechanic is at any time you can declare you flee and you suffer a campaign loss (something bad happens). They wanted to run so i told them what the campaign loss would be (potion ingredients). They were unwilling to accept this so everyone else fled into the sewers and the last guy got captured by the dragon.
Also this was just the most recent example that led me to seek help. There are a fair few other examples that i could also list.
P.s. also the fight the were fleeing was a semi random encounter caused by them choosing to harvest bone marrow (long story) at noon in the middle of a city moments after a very loud public fight. Also i personally think they could have won it just would have been hard.
I have to remind my players that even in their fav fantasy, the heroes retreated often (LotR).
"Do you expect to be able through force of arms to win every fight you get into?" might be a question to ask.