I have created a rulesheet which has a filter on a single field.
In this rule is a single unconditional statement field1 = field2-> avg.
Field2 is displayed as output in rule messages.
When tested field2 obtains the correct value in the test-output.
field2 is displayed as 'null' even if cast from a decimal to a string.
From this question it seems the value is null to start with and is never set.