The bug

Summoning an entity with a formatted custom name will still display the nameplate using default font and style. This also affects mentions of the entity in chat.

How to reproduce

Summon a mob with a name using the minecraft:alt font, or with bold, italic, strikethrough, underlined, coloured or obfuscated characters:

summon minecraft:cow ~ ~ ~ {CustomName: '{"text": "Joe", "font": "minecraft:alt", "bold": true}'}

The nameplate should use bold enchanting table characters, as it would in a tellraw message, but only dislays 'Joe' with default formatting.

Edit: I found that this is already reported for coloured names specifically at MC-181418.