Can't add custom nbt tag to entity

I can't add custom nbt tag with summon nor data merge.

example:

/summon minecraft:chicken ~ ~ ~ {CustomName:'

{"text":"a"}

',aaa:1s}

There is no aaa nbt tag when i used:

/data get entity @e[type=minecraft:chicken,sort=nearest,limit=1,name=a]

Data merge also doesn't work.