Blue
Ravenous Bugblatter Beast of Traal
Telling the players this type of information sounds like really playing the numbers to me.
How do the PCs (let alone the players) know the difference between a CR 5 monster with 80 hits points and no damage resistance vs. a CR 5 monster with 40 hits points and damage resistance? From the perspective of the PCs, both of these monsters have taken the exact same percentage amount of damage with a 10 point non-magic weapon attack.
Except that Hit Points don't exist in the narrative. Let's try these:
"Your swing at the twenty foot giant hits with a nice meaty thunk. It's as nasty a wound as you've delivered but it doesn't seem to inconvenience him."
"Your swing at the animated statue sparks off. Even though it felt like a solid connection, it barely chipped it and didn't inconvenience it."
"Your swing at the gnoll chieftain carves between it's ribs. It was a solid hit, and it stumbles with the pain. It's insane laughter now bubbles with blood, but with a delighted shiver of pain it manages to straighten up and close."
So, which of these was the 15 HP hit vs. 160 HP foe and which was the 15 HP hit vs. the 80 HP foe with resistance. Pretty easy to tell, even though it's the same percentage damage. Your PCs know what they can expect their weapons and spells to do - if it's less (or more), they can tell.
Just like the PCs (and players) should not know how many current or total hit points of damage a given monster has (with the possible exceptions of half damage as per the PHB, or nearly total damage), they also shouldn't know whether a monster has resistance or not (shy of a knowledge check).
This seems the same as saying "As the PCs should not know what color the house is, they also shouldn't know if it has a chimney (shy of a knowledge check)".
Damage and wounds have in-game narrative effects. They are absolutely visible to the PCs. Actual numbers - no. Those don't exist in the narrative. But if a ball of fire washes over a monster and it's not singed and blackened like expected, then it's on the DM to relay that information to the players just as much as if it has a vulnerability and it takes more damage then expected from it. If you'd like that as a rule, 5e PHB pg 4: "3. The DM narrates the results of the adventurers' actions".
If I Fireball a 200 hit point monster for 25 points, how exactly do I know that he took 25 (no resistance, no save) or 12 (resistance or save) or 6 (resistance and save) points of damage compared to a monster with 50 hit points, 100 hit points, or 500 hit points? The monster took some damage. The player might know that the monster saved or not (depending on table), but the PC should be mostly in the dark. The description "he doesn't appear to be as hurt as you'd expect him to be" is just as applicable an expression to the 25 point Fireball against a 500 hit point monster as it is for resistance.
That's because "he doesn't appear to be as hurt as you'd expect him to be" is only half the descriptor. The other part is about the blow you landed. You wouldn't tell a player "you attack didn't seem to hurt it" but hide that they missed the AC, you'd tell them "you missed".
Really, this issue sounds like the DM isn't doing their job of narrating what is happening in sufficient detail that the adventurers would notice. Every case you've brought up has things that the PCs would be in position to observe but instead the feedback on the monster's health instead of on the PC's actions. If the DM focuses on describing the results of the actions instead then the player gets the information their character would have.