Steps to recreate

• Name 2 blank banners simultaneously in an anvil (so they stack)
• Place one of them on the ground
• Break it and pick it up again
• The banners no longer stack

Also works with non-blank banners but these wouldn't stack anyway because of MC-251509.

Cause

Before placing, the named banner has the following NBT data:

{RepairCost: 0, display: {Name: '{"text":"Test"}'}}

After placing and breaking, the banner loses the RepairCost tag:

{display: {Name: '{"text":"Test"}'}}

The items' NBT data doesn't match so they no longer stack.

This bug may seem the same as MC-251509 but they are not caused by the same thing. MC-251509 is caused by the id tag disappearing when placing and breaking. This bug is caused by the RepairCost tag disappearing, a tag only appearing on named banners.