Inheritance and static fields

I have those classes which work in debug build variant but when I enable minify it doesnt work:
interface sample ()
Class One implements sample (){
static field field1
public One()
field1= this;


public static One getInstance()
	if ( field1== null) {
		field1 = new One();
	return field1;

class child1 extends One{}
class child2 extends One{}

in runtime I use child1 and create and object
and later a service asks for the object

class Service {

field= One.getInstance()

The thing is no matter what I keep in the rules file
for Service always write something like v5.getInstance() and create a file called public static v5(){} and create another instance and the object info doesnt exist.

It is difficult to explain, I hope something Ive had a similar problem