Another thing I'd like to point out.
Reloading the map will extend this bugged limit. I have continually re-loaded my map over the past several weeks. Here is a old screenshot showing the previous limit:

And while I haven't made huge progress believe me, I've extended the landfilled area well beyond where the game wouldn't let me before.

In all situations, I've been able to extend the limiting boundary by simply just saving the game and fully reloading it. And continue landfilling and expanding the land available.
As we can see here, the object in the way perfectly contours to the land which I've created... reloading the game gives me another 3-4 tiles of room before I'm limited again.
188989573-fb6dda61-474a-453b-b91d-1f0759124afe.mp4
Here are two save files. They are separated by a reasonable amount of time. The old picture I posted above is from weeks ago and I don't have saves from that long ago to have a larger contrast, sorry.
New Mappy.save
New Mappy.save
Files not imported: New Mappy.save, New Mappy.save, 188989573-fb6dda61-474a-453b-b91d-1f0759124afe.mp4