i dont know if this helps but r169 had a new feature that destroyed everything on export.it created huge seams.
i made a bug report about it and in r170 ashley reverted the export process to r168 and everything is ok again.
also the good news are that he finally fixed the tilemap seams under some circumstances. please refer to the changelog of r170.you just have to select the seamless mode from the tilemap properties on the left of the editor.
for me ,for the first time ever i have no seams with the tilemap object and i am super happy about it!
but if for any reason you still have seams another way to fix them is to add your tilemaps as sprites and then overlap them by 0.5 of a px .
lets say you have 128x128 tile-sprites.
you make your grid 127.5x127.5 and you snap them.