They usually have a Cleric that heals them. That means they've established a formula on how they interact with enemies and how they perform in combat that takes that healing into account.
But apparently, they've become tactically complacent because of it and did not spend the proper amount of time working how just how they needed to now act without their constant stream of healing spells behind them. Thus, they played poorly this session and it didn't feel good. They had to run away, something that it sounds they were not used to doing (and which as you say, left a bad taste in their mouths.)
In my personal opinion as a DM... I think that's GOOD. And that you absolutely did the right thing by not letting the missing cleric's healing spells be available.
Too many times players become complacent, they expect the same things to happen each and every combat, and they complain when it doesn't. You know what I say? Too bad.
In my games, it's my players of Ranged PCs that are the biggest culprits... who bitch and moan when enemies occasionally actually get up in their grill and attack them in melee, then the players clutch their pearls and freak out when their PCs actually gets knocked out. An occurrence which their melee friends deal with all the fricking time and we
never hear them complain about going to 0 HP. But my Ranged players are all throwing their pencils into the air moaning "Well, I'M DEAD!"... completely ignoring the little problem that they still need to
fail 3 freaking saving throws first, and there are still two or three other party members
with healing available to get them back on their feet. Just like they do for the tank almost every single fight.
Little bitches.
Okay... sorry for that tangent rant

... but the point is the same. If they're complaining because things happened that don't ordinarily happen because they didn't adjust how they played the game due to a missing player? That's on them. Suck it up. Learn from it. And next time, maybe they'll be a little more smart and a little less whiny about how they play.
Good luck!