When you built the add-ons for your "Shed" did you build the floor tiles disconnected from the "Shed" floor tiles and get a pop-up to name the new building/construction, and you entered a name like, 'House' or 'Bedroom'? If you did, Mirel won't see that new area as "Shed" but as whatever you named it.
If you didn't, then it's probably a bug. But if you did, then I can possibly give some advice, assuming what I know hasn't changed or was wrong to begin with.
Mirel will stick to the designated "Shed" area unless you undo the entire flooring on the add-on and rebuild it outwards from the Shed flooring to keep it connected. If you remove the "Shed" flooring (also under the walls) and make it something else like, "House" before she moves in, I don't think it'll cause any issues.
If you completely remove the "Shed" flooring area while she's living in it and expand the other area into it, she'll probably complain about not having a place to live, since the original area is gone, and will probably leave back to Thalassar. Not sure how well she'll handle it, but you can try to get her to move out and into the other side of the "House" and then remove the "Shed" side altogether afterward?