The bug

I’ve noticed there are two ‘types’ of patterned banner that are unable to stack with each other. The first ‘type’ of patterned banner is a banner that has never been placed down before and has only existed in item form. The other ‘type’ of patterned banner is a banner that has been placed down and broken, even if they were previously stacked, they would now be unable to stack.

To reproduce

Make 2 identical banners (pattern doesn’t matter as long as they are identical), then place one of the banners down. When broken, the banner that has been placed is unable to stack with the other banner, they are only able to stack once you have placed down and broken the other banner too.

This works for any banner pattern.

This does not work for blank banners.

This has existed since 1.8 and is still happening in 1.18.2

Cause

From MC-256342: The banner, when mined, loses its BlockEntityTag "minecraft:banner" id, causing the NBTs of the items to no longer be identical and thus no longer stack.

Code analysis

Code analysis by Kevinthegreat can be found in duplicate MC-256994.