Disappearing bookshelves/ cakes

Firstly, I’m not sure if I selected the correct “affects version/s” because I’m unsure what version is on my iPad. 
I’ve had a problem twice now where I was working towards getting 18 bookshelves in order to max my enchanting table. The first time it happened, I had all but 3 bookshelves placed around my table, and left it like that to procure resources to make the last 3. When I returned, 9 of my shelves were missing, and had not become item blocks anywhere. So, I went off and worked towards more books, and got up to 14 shelves placed, when I returned this time, 7 shelves were missing.