Recently reported the API for setting shield base color not working ([SPIGOT-7809] Can't set shield base color - SpigotMC JIRA), which was fixed, but it seems coloring shields via crafting is broken now, as combining a shield with any banner just makes a white shield.

Additionally it seems some extra data is added by Spigot there? a Shield crafted on vanilla is

{count: 1, Slot: 0b, components: {"minecraft:base_color": "red"}, id: "minecraft:shield"}

(just the base_color component)

But one crafted on a Spigot server is

{count: 1, Slot: 0b, components: {"minecraft:base_color": "white", "minecraft:banner_patterns": [], "minecraft:block_entity_data": {components:
{"minecraft:base_color": "red"}
, x: 0, y: 0, z: 0, id: "minecraft:banner"}}, id: "minecraft:shield"}

So would be nice if that could be fixed in the API & normal crafting (I assume you'd need to have some special case logic in BlockStateMeta for shields that applies the base_color component instead of the block_entity_data component? as changing the meta type would break API, unless there's some way to do it with commodore or something like that)