I have this issue too.
What it looks to me is that, after you using the Paint mode, after going back to the Sim creator, the game is saving the texture of the work you made into a low quality or bad format which make the texture looks weird, but low quality means less file size which also makes loading times faster.
But not only that, the UV Mapping grid edges of the model are not being colored, but stays with the same color after you use the bucket tool or "Fill Color" or the base color it had before you start painting, that's why you see the lines. The game is saving the texture in low quality which makes the colors by the grid edges to stretch out over the model.
I don't think this has anything to do with our graphic cards, nor the game, but the texture itself. I suppose the develop team could add a new feature which makes the color you paint by the edges of the model to stretch out even more on the texture file, just a tiny itch beyond the edges of the UV Map grids. That could fix the issues here. Or that they letting us to save the texture on our computers that we can open with our paint programs and stretch out the colors that way.
These keeps happening to me. Is there a fix yet?
I have also this problem when i painted black cat with white. There is very disturbing black lines especially on the middle chest and face☹️ I wanted to make my cat in sims that i could have nice shot whith her on my stream panel, i dont know what to do now because i dont know how to photoshop:/
