Closing as not a bug, this is just a re-hash of the "adjacent objects not seamless due to floating point rendering" gotcha that lots of people run in to.
For adjacent objects to be exactly seamless, you must use letterbox integer scale, point sampling, and pixel rounding. Any other settings are liable to show seams between objects due to floating-point positioning or scaling.
Spritesheeting changes the way object edges blend: normally they clamp to the object edge color, but on a spritesheet they are surrounded by transparency so fractional positions can slightly blend with transparency. This is a normal result in computer graphics, and the tilemap object is similarly affected.