When you really think about it, the trust system is weird and I say this despite the fact I like it
Because Ryne is in the system, they just had her stop at Shadowbringers and I assume it's due to her being limited to the First but as you mentioned you can have characters do the first dungeon despite not showing up until much later

If I had to guess, it's probably because of the duty support system being added so now Trusts will likely just be to level the scions up and put them in new outfits