I would lean towards one of the pieces being cupped and the surfaces not contacting fully. URAC will hold that gap, TB3 wont.
This is it. Bamboo backs are crowned and TB II has enough water in it that the bamboo can absor someand the edges will lift. This is why I always use rubber innertube clamps when putting on a bamboo backing, and lots of them. AND, I Like the bamboo to be about as wide as the wood, so the edges really hold dow,
I had the same problem you did, when I used C-clamps all along.