To confirm the issue, I created a user through the API on 3.4.0.beta3-dev and it returned user_id in the response.
{
"success": true,
"active": true,
"message": "Your account is activated and ready to use.",
"user_id": 3
}
I did it again after upgrading to 3.4.0.beta4-dev and user_id is then missing:
{
"success": true,
"active": true,
"message": "Your account is activated and ready to use."
}