I think it looks very nice and is certainly playable as is. But to address your question:
Are the edges of the foamcore bent down at the join? Sometimes that happens during cutting or when burnishing the edge of the print-out onto the underlying foamcore. If that's the case, you may be able to fix it by carefully elevating the bent-down edge. Another possible problem could be that the slots you made for the biscuits are not in the same plane, if one side has a different angle than the other, that could also cause this problem.
I've used the biscuit method in the past but I kept losing the biscuits. Now I use either an old magnetic dry erase board or a 2x2 foot wooden board painted with magnetic paint, and apply strips of cheap craft store magnetic strips beneath the tile pieces. The tile pieces stay in place pretty well on the magnetic board and there's no biscuits to lose. A more expensive option I tried was to print out the floor tiles onto magnetic photo-paper. Works well but was kind of pricey, even in 15mm scale.