Drawers can be held in stacks of up to 64 items when they contain items inside them. The only caveat is that the drawers must possess the exact same upgrades and quantity of items to stack. Regardless, this oversight allows players the ability to carry millions of items at a time, theoretically.

Although this issue seems quite insignificant, it is exaggerated by bundles. Because a bundle can hold up to 64 stackable items in one slot, this includes drawers, regardless of their contents.
Because of this, drawers and bundles not only behave like day-one shulker boxes, but can hold substantially more items than shulker boxes can.

TL;DR: At minimum, bundles should not be able to store drawers containing items.

Fixed in latest releases for 1.20.1, 1.21.1, 1.21.8. The update:

• Prevents filled drawers from stacking
• Blocks drawers from being placed in bundles or shulker boxes (including empty ones but not a simple effort to treat them separately)
• Filled drawers can't be placed in other drawers (configurable)