There seem to be 2 instances of how this stack “error” happens; the first is indeed when the item in the stack doesnt have a price.
This can happen with any item, even outside a stack, if it doesnt have a price. Items without prices wont be displayed when just looking at the stand from outside. Example would be if you place yams and meat in a shop stand but only price the yams. The stand will display only yams, but when interacted you can see the meat but cant buy it (seems to work as intended).
The second error occurs only in smartstacks, when the smartstack contains 2 or more items and at least one of them dont have a price. Example: put those yams and meat from example 1 into a single stack and only put up a price for the yams. Again from the outside you can only see a yam displayed. Now upon interacting you see a smartstack but you will be unable to open the stack and pick out the yams from the meat.
Error 1 is fine, you can get what you want and you can see stuff you cant buy. Error 2 is frustrating because you can see what you want but cant get to it at all
TL/DR; Sellers make sure your smart stacks are “pure” before dropping them in stands, 1 item only per stack.