**Easy Answer**

The description states *(emphasis mine)*:

>While you are wearing the mask and attuned to it, you **can** access the following properties.

This means you do not have to access them and can essentially turn them off only to re-enable them, so even though Damage Absorption alone is not retroactive, you can refresh it anyway.

**The Full Story**

If the mask said something along the lines of: "While you are wearing the mask and attuned to it, you gain the following properties" this would be entirely different and you could not gain temporary immunity. Compare to the Iron Mind feature of the Gloom Stalker Ranger: 

> You gain proficiency in Wisdom saving throws. If you already have this proficiency, you instead gain proficiency in Intelligence or Charisma saving throws (your choice).

This feature would not retroactively update if, for example, you took the Resilient feat. The ability to switch the properties on and off at-will give the Dragon Mask greater versatility.

*Added note: This also means that once you gain immunity from the feature, this immunity is maintained as long as you wear and attune to the mask and continue to benefit from the properties since nothing in the text says the immunity you gain is removed.*