The BASH below is not specific to Flask, but the MySQL probably is.

Send to this URL to see just what you have actually sent to help any debugging.

https://httpbin.org/anything

curl -i -X POST \
-H 'Content-Type: application/json' \
-d "{\"auth_token\":\"${AUTH_TOKEN}\", \"user_id\":\"${EMAIL}\", \"file_ref\":\"File.odt\", \"uuid\":\"${UUID}\"}" \ 
http://192.168.11.7:5000/api

Get output from MySQL in the form of a Tuple and return it without any html encoding, ie. just as a json object. It has a 200 hard coded, but it might be wise to check that the result from MySQL is sensible and justifies a 200 return.

mycursor = mydb.cursor()
mycursor.execute(sql)
user_id = mycursor.fetchone()  # returns a tuple
lookup_userid = {"userid":user_id[0]}
 
response = make_response(jsonify({"id": str(lookup_userid) }), 200,)
response.headers["Content-Type"] = "application/json"
return response