5.5 Truesight vs. Doppelganger

The title says it all. My party and I have a difference of opinion regarding a doppelganger's true form and another creature seeing it with truesight.

My thoughts are thus: 

Truesight

If you have Truesight, your vision is enhanced within a specified range. Within that range, your vision pierces through the following:

Darkness. You can see in normal and magical Darkness.

Invisibility. You see creatures and objects that have the Invisible condition.

Visual Illusions. Visual illusions appear transparent to you, and you automatically succeed on saving throws against them.

Transformations. You discern the true form of any creature or object you see that has been transformed by magic. <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Ethereal Plane. You see into the Ethereal Plane.

It is the indicated line that we disagree on the ruling of. Nothing in the doppelganger statblock, nor the mimic or werewolf statblocks for that matter, states that the shapechange is magical in nature. With that, would true seeing actually see the creature(s) for what they are?

Any help would be appreciated.

I agree with you, but it's not actually clear. According to their description, Doppelgängers are "supernatural beings". Does that make their shapechange magic? Maybe!

Mimics are probably non-magical. Werewolves are probably magical, because the curse is magical in nature. Doppelgängers are <shrug emoji>. 

I made the decision I did because they're supposed to be a threat, and easy outs reduce that threat. (It's a clearer call in my current game, where I explicitly defined them as natural beings -- basically advanced octopuses; but that's house rules.)

In 2014, truesight could explicitly "perceive the original form of a shapechanger or a creature that is transformed by magic". In 2024, reference to shapechanger was removed. Note that some shapeshifting creatures (such as an ancient gold dragon) do explicitly change shape with magic, but many others do not.

I don't remember this being called out as a change in 2024, but it's definitely not the same wording so I'm inclined to consider it a real change.