A spawn egg with entity tag command_block_minecart spawns a command block minecart with the name defaulting to "@".

If you right click that spawn egg on a monster spawner or trial spawner, the minecart that spawns will appear to not have the "@" name. Only once you reload will the name show when you hover over the minecart.

Reproduction Steps:

1. Run a command to get a spawn egg with entity data command_block_minecart, such as

/give @p breeze_spawn_egg{EntityTag:{id:"command_block_minecart"}} 1

2. Right click it on a block to spawn it. You will see it has the default name "@" when you hover over it.

3. Right click on a monster spawner or trial spawner with it and wait for it to spawn the minecart

4. When you hover over the spawned minecart, it appears to have no name

5. Quit and reopen the world and the name now shows