Does adding the Damage Absorption trait to a not-immune creature change its CR?

Question

The DMG has a "Monster Features" table (p. 280), containing traits that can be added onto monsters for customization purposes. One of the traits with no effect on CR listed is Damage Absorption (it gives the flesh golem as an example of a monster with the trait):

Whenever the [creature] is subjected to [type] damage, it takes no damage and instead regains a number of hit points equal to the [type] damage dealt.

It does seem not-CR-changing to add that Absorption feature onto already-immune creatures (ex: Cold Absorption on an Ice Devil), and all official creatures that have a Damage Absorption also have an Immunity of the same type (ex: Iron Golem is both Fire-Immune and Fire-Absorbent).

But... what about adding Damage Absorption to a creature not already immune to the damage type (e.g. adding Fire Absorption to a Hydra)? Although adding the feature supposedly doesn't change CR by itself, does it imply a de facto immunity, thus changing the CR?

Answer 1

It may result in an increase to CR

The DMG leaves a lot of ambiguity in the section on creating a monster. I suppose we ought to break this into two sections:

You should consider Damage Absorption to imply immunity for the purposes of calculating CR

Damage Absorption may not explicitly grant creatures immunity, but it replicates the effect (in fact, it is strictly more powerful). CR calculations are already murky, but we should at least be able to assume that the effect of an ability matters more than its name. Therefore, since it gives all the benefits of immunity, and has no added drawbacks, you should count it as at least equivalent to immunity for calculating CR.

The DMG likely assumes that a creature with Damage Absorption against a certain damage type already has immunity versus that damage type and therefore does not give it a CR increase. Additionally...

But it does not necessarily change the CR

Remember that the DMG section on calculating the effect of resistance/immunity on CR (p.277) says that

If a monster has resistance or immunity to several damage types- especially bludgeoning, piercing, and slashing damage from nonmagical weapons-and not all the characters in the party possess the means to counteract that resistance or immunity, you need to take these defenses into account when comparing your monster's hit points to its expected challenge rating.

So the specific immunity matters in determining whether a CR increase is justified. For example, giving a monster immunity to acid damage is unlikely to justify an increase to CR, as acid damage is uncommon and the party is extremely likely to have alternate damage types. Therefore, acid immunity (and, in the same vein, acidic Damage Absorption) would not justify an increase to CR.

Since adding an immunity does not necessarily result in an increase to CR, it makes sense that they would not put a CR increase as a direct result of Damage Absorption; rather, this would be something the DM has to consider, based on the guidance given in the quoted section.