The first kombucha leather I grew was in The New School science lab with the help and supervision of the lab manager, Marcus. We followed the instructions word for word but doubled the liquid mixture and split the SCOBY between another student and myself. We let the culture grow for about two weeks. Before we pulled the kombucha leather out of the container, we noticed that it was very bubbly and thin. As we pulled it out, it began to rip almost immediately. We let it dry on cellophane for two weeks hoping the kombucha leather would dry normally, but it was very brittle. You couldn't pick it up without pieces breaking and flaking off. Marcus made a spray out of beeswax, coconut oil, and mineral oil and I spray some onto the kombucha leather. This spray is supposed to make the kombucha leather more flexible, but my kombucha leather remained brittle. The student that I split the culture with had the same results as I did---bubbly, brittle, and thin. For future reference, don't split the SCOBY because the kombucha leather won't grow properly.
I grew two more kombucha leathers on my own and the results were a bit better. I didn't split the SCOBY this time, and instead had a SCOBY per container. For one batch, batch A, I didn't alter the instructions at all. For the other batch, batch B, I left out the apple cider vinegar. I let these grow for four weeks. Batch A grew normally but was still fairly thin. Batch B, on the other hand, didn't grow properly. Instead, pieces of the kombucha leather were floating in the container and it seemed like mold was growing on it because there were black spots. This is only one test, but it seems like the apple cider vinegar played a part in forming the kombucha leather as well as possible sanitary issues. Further tests are needed to confirm this.