There are two elements to trusts: the story-relevant party when you arrive at the dungeon in the course of the plot, and the "avatar" party members that you have to level up afterwards.

While you're levelling the avatars, Ryne's presence in Holminster is every bit as lore-breaking as it would be for her to participate in a Source dungeon. She simply wasn't present there and then.

Most likely the avatar system will remain in place beyond level 80 and you'll be able to continue levelling Ryne('s avatar) alongside all the other trust avatars - both the ones you've raised from 70 and at least one new one starting at 80 - even though she is unlikely to be present as an active character in the story dungeons.