Found the solution, i.e. if you use the following code snippet you only get one warning:
122 String contactName = contact.getName();
123 name = name + contactName.substring(0, 1).toUpperCase();
-> Implicitly using the default local is common source of bugs: Use toUpperCase(Locale) instead; Contact.java; line 123
Also if you're using something like:
123 name = contactName.isEmpty() ? "-" : contactName.substring(0, 1).toUpperCase();
you'll get the same two warnings as mentioned before !?!
Probably has to do with building/parsing the expression tree IMHO.