Skyrim Special Edition – Fix for Crashing When Leaving or Approaching Whiterun on Xbox One

It sounds to me like one of the characters isn't following their script correctly here. Although I've yet to hear about this quest bugging out. You may have done something before giving Septimus the lexicon that bugged one of the character scripts.

The best advice is reloading a previous save, even reloading the auto-save right before entering this area should reset the script for you.

To find Lucia, or really any missing NPC:

1. Figure out their ref ID (not base ID). For a lot of NPCs, you can find this on their UESP Wiki page. The page for Lucia says that her ref ID is xx003F5E. Replace the "xx" with wherever your DLC is in the load order. Hearthfire is #5 in my load order, so Lucia's ref ID for me is 05003F5E. UESP has an article on finding the first two digits of the ref ID.

   If neither UESP nor the Elder Scrolls Wiki have the ref ID, you could load a different saved game and click on the NPC with the console open. That will show their ref ID centered on the console window. Or you could look in the Creation Kit, if you're desperate.

   If you type help Lucia in the console, that will give you her base ID, which won't do you any good unless you want to spawn another Lucia. I tried that, but adopting one of her clones didn't seem to work: she agreed to be adopted but never went to my house.

2. Type this in the console: player.moveto 05003F5E (or whatever the ref ID is for you). That should move you to wherever Lucia is.
3. If you still don't see her, type this in the console, just in case she got disabled somehow:
   prid 05003F5E
enable
4. If the NPC is stuck/won't move (which might explain why a follower stopped following you), click on them while in the console and type moveto player. Or sprint into them a few times and try to stagger them to some other location.
5. If you found the NPC you're looking for but they're dead, take any important items off of their body, then click on them while in the console and type resurrect.

Hopefully, by now you're standing next to the NPC and they're alive.

The answers to this question are also relevant for finding NPCs: How do I find my follower if and when they leave me?

You mentioned that you wanted to unadopt Lucia, so check the answers to this question for how to do that: How do I unadopt my children? (preferably without killing them)