Maybe your style of encounters leads to frustration due to lacking sense of achievement. Okay, they win, yes, but only with huge effort (80% resources*), and then for all that trouble, all they have achieved was neutralizing some goblins or whatever. That doesn't even earn them bragging rights.
Imagine being in the PC's shoes, as they return to town after a hard adventuring day:
Barkeep: "Whoa, you look pretty beat up, what ever happened to you?"
B: "What, Goblins did that to you? Are you messing with me?"
P: "Yah well, they were really nasty Goblins and they used the terrain to their advantage."
B: "Riiiight, the terrain. Uh-huh."
P: "We did kill them eventually!"
B: "And do you want a medal for Goblin-slaying now? Tell you what, here's a cup of warm milk on the house for everyone. Also, I have a cellar full of rats, that might be more up your alley."
Now imagine the same scene, except the party killed a band of Trolls or Fire Giants or whatever sounds tough at their level. I don't think I need to spell it out, but the Barkeep's reaction would be different.
So, regardless of whether any such scenes ever play out at your table -- something like that is probably going on inside your players' heads. Hence they get mopey.
*) come to think of it, strictly speaking "80% of resources" cannot mean "80% of HP _and_ 80% of spells", much less "and 80% of consumables". After all, HP need to be restored, and that will probably require more magic resources (like the next day's spell slots). Sure, casting Cure spells in downtime is practically free, but keep in mind that if restoring those HP takes, say, 40% of another day's spell slots, that adventuring day actually cost them 120% of their daily resources. Ofc I don't know if you handle it that way or not, just wanted to get the thought out.