Not sure if it's really enough to be a trope but the straight Li's in DA seem to be able to be in more of a committed state than the bi Li's.
Alistair can marry the female Warden and we know that in that situation at least they stay together through to DA2. Also if you are in a relationship with Alistair and he is king even if you aren't queen) it is shown that him and the pc are still together.
Morrigan and the male Warden can reunite in the Witch Hunt DLC. DA:I could rectify this as she seems to be Wardenless but the Warden have vanished regardless of romance so idk, it depends on how it plays out.
Sebastian can marry female Hawke and we know from DLC that this actually did happen, I know it's still just a chaste marriage but it is a commitment.
Zevran is Wardenless in DA2 although he was meant to be still with the Warden if not for that bug, they do seem to be apart anyway.
Leliana is Wardenless in DA2 and DA:I
Both can still be in love with the PC but neither seem to be with the PC.
All the DA2 romances can end in commitment but we don't know if it sticks, the only one that stands above the others is Sebastian's because of the Marriage thing.
So it's a theme but I am not sure if we know enough to call it a trope. DA:I could change it I guess.