When handling Periods in InfixOpNode [1], there is getMonths() method used, which returns just months part of the Period. There should be call to toTotalMonths() so also years are incorporated in the result. I will provide PR for this.

[1] https://github.com/kiegroup/drools/blob/a631144cf7c5854775253ef619dd720405123800/kie-dmn/kie-dmn-feel/src/main/java/org/kie/dmn/feel/lang/ast/InfixOpNode.java#L257