This happened to Mist and myself on two different occasions. Mist was building a bridge section from my cart and had to put the rock shards on his person to build, but it pulled them from the cart for me. Today I was reinforcing the floor, and had to put the ribbons on me to do it... yesterday I was able to do it from the cart. Not sure what is happening, Mist suggested letting you know. Have a good day!
I tried to reproduce this on my test server but wasn't able to.
Hm what were they stored in? It will pull from crates but not BSBs.
Also you said that mist was not able to build from your cart... Were you in your own cart when you failed to reinforce? Maybe it's something wonky with permissions or ownership of the items? If it happens again try changing them or transferring ownership and see if it changes anything.