I am running the game on Linux, Nvidia drivers, ... (if you need to know more, just ask)

Mostly, the tiles are adjacent, neighboring smoothly, but sometimes, I see super thin black borders around them, not on all 4 sides (although I have seen that too), but usually only on 1 or 2 sides and usually, the tiles below or above have it too.

It looks like some floating point precision errors on the gpu. I don't know how you implemented the rendering so I can't tell you more. Maybe you can solve it by adding some mathematical correction factors or by doing relative rendering instead of absolute rendering. Or maybe you can solve it by just clearing the back buffer with a brownish color so nobody notices anymore.