I think that the issue came from another code in the child's theme functions.php. You have used "$user_ID" instead of "$user_id". Check this screenshot hidden link
I fixed that part of code. Then I installed the "WP Mail Logging" to be able to catch the sent emails, and I run a test. The email has been sent to the selected user in the field "user id". Check this screenshot hidden link
I'll let you test it from your side too and let me know if it is still not working.