When using glass bottles on beehives or nests the last glass bottle will not stack with the other ones and will instead form its own stack. You can combine the two stacks together but originally they are separate.
is duplicated by
MC-263738Bottles not stacking after emptied (or filled in the case of honey bottles)
Resolved
relates to
MC-112082Dragon's breath created by last glass bottle of stack is not merged with existing dragon's breath items