Are there cases where Retreating awards items, other than Explore's Minibosses?

Generally, retreating from a battle rewards you nothing at all, so it's even worse than losing a battle. In my opinion, the only plausible reasons one might retreat are:

1. They're stuck in a battle that's likely to run too long. This happens when dealing with infinitely respawning teams (like bandit spam teams, or Life and Death teams (which respawns giant spiders endlessly)), or infinitely healing teams (like Guardian Crown teams).

2. You are chasing the PvP leaderboard and you need your battles to be as fast as possible. Often this means that if your starting board is unfavourable, you'd retreat (with no rewards) and try again, just to save time.

Don't retreat from a Mini-Boss battle just to avoid the Mythic Boss battle. (You can simply do an explore run in another kingdom, and "consume" the mythstones in the second kingdom.)