PHP function http_build_query(); would return different string for this array. It won’t remain readable like an array but, I believe, it will work as parameter to WP_Query.
build_query is the WordPress equivalent for the same functionality.
Turns out that build_query is a bit different from http_build_query.
http_build_query returned date_query%5B0%5D%5Bafter%5D=24+hours+ago and build_query returned date_query%5B0%5D%5Bafter%5D=24 hours ago. I guess, later would work for WP_Query.