invisible var content to echo when not quoted - why

[Alex fxmbsw7 Ratchev]

declare 's=1 foo["$s"]=asd' ; declare -p s foo ; echo $s ; echo "$s" ; echo
fin

declare -- s="1 foo[\"\$s\"]=asd"
bash: declare: foo: not found

1 foo["$s"]=asd
fin


notice the empty line
i know foo unset is my mistake