For races with variant options, the character builder displays a short description of the base races traits. This is because unlike races with subraces, a variant race can replace any and all traits of the original. This means that before the Mark of Detection Half-Elf was released, D&D Beyond had no idea if there even would be a variant half-elf, nor what traits would get replaced. There's no way to preview the base race and all possible variant racial traits, so it just displays the base race information.
The Mark of Detection Half-Elf states
Racial Traits
+2 Charisma, +1 to Two Other Ability Scores, Darkvision, Fey Ancestry, Skill Versatility
yet when you choose the race, it comes up with the following (which, incidentally, is applied correctly)
Ability Score Increase
Your Wisdom score increases by 2, and one other ability score of your choice increases by 1.
For races with variant options, the character builder displays a short description of the base races traits. This is because unlike races with subraces, a variant race can replace any and all traits of the original. This means that before the Mark of Detection Half-Elf was released, D&D Beyond had no idea if there even would be a variant half-elf, nor what traits would get replaced. There's no way to preview the base race and all possible variant racial traits, so it just displays the base race information.
Find my D&D Beyond articles here