Is there ever a case where a creature gets a saving throw against the "Sleep" spell?

Question

The sleep spell (PHB 276) states:

This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect.

This and other similar spells like color spray affect a pool of hit points directly, not dealing damage and encountering potential resistances nor requiring a saving throw that may be given advantage via mechanics.

There are creatures that explicitly give advantage on saving throws against being put to sleep magically, though, such as the Bugbear Chief (MM 33), which has the trait Heart of Hruggek:

The bugbear has advantage on saving throws against being charmed, frightened, paralyzed, poisoned, stunned, or put to sleep.

(Emphasis mine)

or the Balor (MM 55), which has the trait Magic Resistance:

The balor has advantage on saving throws against spells and other magical effects.

(Emphasis mine)

Sleep in this case comes from a magical source, so I believe it falls under the category of a magical effect.

Given these specific wordings, does the general behavior of the sleep spell change to fit the circumstance, giving the targeted creature a saving throw against the caster's DC? If so, what type of saving throw? Or does the spell simply ignore these traits and proceed to affect the hit point pool directly?

Answer 1

RAW, no.

Or, answering the body question instead, the spell ignores these features.

Spells do (only) what they say. The spell does not force a saving throw. For the monster's feature to be a "Specific beats general" case, it should explicitly state something along the lines of:

If this creature is affected by the sleep spell, it gets a X saving throw. The consequences for failing are Y. The consequences for succeeding are Z.

Neither of them do, and, as far as I'm aware, no creature or class has a feature worded like that, i.e., no feature states that "they get a saving throw" in any way. They simply have advantage against saving throws that already exist, which sleep doesn't provide.

A similar case is

The mirror image spell has no effect on magic missile, which doesn't involve an attack.

So, we could answer with the same logic:

The bugbear chief's feature Heart of Hruggek has no effect on sleep, which doesn't involve a saving throw.

Additional conjecture for my answer is that you would have to adjudicate what type of saving throw? Is it Wis? Con? What happens if the creature succeeds the saving throw? Is it simply not affected? Does the HP from sleep's HP pool that would have been used for that monster get reduced anyway? While these answers might be easy to adjudicate, none of them is answered by neither the spell description or the feature. Spells are (supposed to be) clear (that's the whole point of "Spells do what they say"), so, if you have to adjudicate so many things, this is probably not how the effect should be resolved.

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As for "Why does the Bugbear Chief have a feature like that then?", I've created this question. NautArch has already provided examples. One of them, the symbol spell, has a possible effect that states:

Sleep. Each target must make a Wisdom saving throw and falls unconscious for 10 minutes on a failed save. A creature awakens if it takes damage or if someone uses an action to shake or slap it awake.