Lodestone compasses are stackable up to 64.

Repro
1. Obtain 64 compasses in creative
2. Use them to right click a lodestone block
3. Observe that you now have 64 lodestone compasses in a stack

or

1. Obtain two different compasses in two different hotbar slots
2. Individually use both to right click a lodestone block.
3. Now drag and drop one of the lodestone compasses on top of the other
4. Observe that they stack