I put this in unity, set it at 16x16 and 16 ppu, and some tiles catch the border from the tile next to it. Most of the tiles don't fit together and have a small gap in between.
Everything is ok with tiles, you have to do something wrong. I just checked in Unity and everything is fine.
We have to do something wrong to make it work properly? That doesn't really make sense.
If you mapped sprites properly and created tiles out of them - i had the same problem. Just set the resolution to specific one (i.e. 640x360 for 16:9 not only by aspect ratio) and it will display properly.
It's due to scaling if you have not locked your preview resolution.
Oh! Turn of filtering (set it to point)