The use of multi-restriction or the use of "in" operator causes a unwanted coersion from String types to decimal types.
For example (Message.message is String and someEntity.someString is String as well):


            Message message = new Message();
            SomeEntity someEntity = new SomeEntity();
	    someEntity.setSomeString("1.5");
            message.setSomeEntity(someEntity);
            message.setMessage("1.5");
            ksession.insert(message);
            ksession.fireAllRules();


rule "R1"
dialect "java" 
salience 2
    when
        Message($message: message in ("1.500") )
    then
        System.out.println("Result for R1 " + $message );
end


rule "R2"
dialect "java"
salience 1 
    when
      m:  Message( someEntity.someString in ("1.500") )
    then
        System.out.println("Result for R2 " + m.getSomeEntity().getSomeString() );
end


Using Drools 5.2 and Drools 5.3 you have printed:

"Result for R1 1.5
 Result for R2 1.5"

... there, was done a coercion to decimal and all conditionals returned true then the rules were fired. If used a single constraint with "==" operator instead of "in", no rule is fired (as expected).

Using Drools 5.0.x and Drools 5.1.x the result is different, only R2 is fired... so there is the same problem, but only for nested objects.
The fix should works for both cases, nested objects and for direct access for properties.

I am not sure if Multi-restriction is a MVEl feature, I think it is implemented by pure Drools parser, so I am not opening a new MVEL issue based on that. However, let me know if it is necessary.