Expected Behaviour

Holding an item and left-clicking on a Bundle:

• puts the held item in that slot and the player now holds the Bundle.

Holding a Bundle and left-clicking another item:

• puts the Bundle in that slot and the player now holds the item that was clicked.

1.21.2 Pre-Release 1 Behaviour

Holding an item and left-clicking on a Bundle:

• puts the held item in the Bundle and the player now holds nothing.

Holding a Bundle and left-clicking another item:

• puts the clicked item in the Bundle and the player still holds the Bundle.

Summary

The expected behaviour (items are switched) is what happens with all other items in the game, including Shulker Boxes. The behaviour observed in 1.21.2 Pre-1 is what should happen when right-clicking. This would match what is already the button to remove an item from a held Bundle. Left-click to move item stacks; Right-click to interact with an item stack.