Icewind Dale: Rime of the Frostmaiden contains a Character Secrets section, one of which is "Doppelganger" :
I’m a shapechanger. I have none of the traits of the race I’m imitating, but I retain my class features, background, and alignment. I have darkvision out to a range of 60 feet. I can use an action to do either of the following at will:
I can innately cast detect thoughts, requiring no components. My spellcasting ability is Intelligence. The spell’s Wisdom save DC is 8 + my proficiency bonus + my Intelligence modifier. I can polymorph into any humanoid I have seen or back into my true form. My statistics, other than my size, don’t change. Any equipment I’m wearing or carrying isn’t transformed. I revert to my true form when I die.
Does this mean that your default race’s traits are nullified when selecting this Character Secret ? Example – a Variant Human Doppelganger wouldn’t have the +1 bonus to two ability scores, the extra skill & extra feat, with that interpretation.