If you look closely at your heightmap you'll notice that the edge heights are completely different (the top edge of the curb is quite low, but the right edge is average) so when you're butting the two instances together the parallax heights are different, causing that seam you're seeing. This is the only conclusion I can…
So, I've managed to keep myself away from Cryengine all these years, but I thought it'd finally be time to give it a proper go. And with that being said I'm completely new to it. I'm building a new scene at the moment and were hoping to pop it all in there, now experimenting with its tessellation and parallax effects, but…