I have had terrain paint move slightly in every sim game since Sims 2, on literally every lot I have ever placed. In my game, this is nothing new. I have seen it since I started playing Sims 4, just before the Cottage Living expansion. If anything, it is less noticeable in Sims 4 than it was in Sims 2 and Sims 3.
I move lots around all the time, both from my Library and the gallery. Some lots shift more than others, but none of them stay exactly the same. On a few lots it is noticeable enough that I have to do touch up repainting. But some are so slight that I can just leave them as they are.
Lots that have to be reoriented are usually the worst culprits for large shifts. Lots that are a different size than they previously were also tend to shift more. But sometimes it is an elevation change on a particular lot, even if it looks flat, especially if the lot is changed from one world to another where base line elevation differs.
I suspect this is simply a matter of the constraints of the game, and an inconvenience that cannot be avoided, rather than an actual bug. It has been nothing more than that, in my experience, across all the Sims games.
