Dragonmark variants/subraces - there are 13 of these presented in Chapter 1. All of them have a racial feature named, "Spells of the Mark" that adds spells to the character's spell list, if that class has spellcasting or pact magic. We haven't been able to model that correctly at this time, but will implement as soon as we are able to.
It appears that the elf subrace ‘mark of shadow’ is not adding spells of the mark to the available spells for a sorcerer multi class.
This is a know issue and is actually one of the first things mentioned in the first comment in the second pinned thread on this subforum
Eberron: Rising from the Last War - Bugs and Support thread
Find my D&D Beyond articles here