Are you aware that in Mine Blocks 1.28 books are only able to be enchanted with unbreaking? This probably has something to do with adding enchantments on balloons.

Yes I was aware :) And it's been fixed for the next update too!
And your guess was right!
It was because ever since enchanted books were added, it wasn't picking a random item group when choosing which type of enchantments to add to the book - it was picking the first item group in the list.
In 1.27, the first one in the list was the helmet enchantments, so there were a lot of different enchantments for it to choose from.
When balloons were added, the first item in the list became the balloon enchantments. Therefore the book only got unbreaking.
Thank you very much for telling me about it though! :D
And sorry for the inconvenience - it really breaks enchanted books unfortunately haha