Given the following rule replicated for enums EDay.SUNDAY through EDay.THURSDAY and
a single DummyTester fact starting with EDay.SUNDAY.
rule "Sunday"
when
    $dt: DummyTester(EDay.SUNDAY == day)
then
    System.out.println("RULE: entered rule: "+drools.getRule().getName());
    modify ($dt) {setDay(EDay.MONDAY)};
end

On 5.1.1 and using HEAD:
RULE: entered rule: Sunday
RULE: entered rule: Monday
RULE: entered rule: Tuesday
RULE: entered rule: Wednsday
RULE: entered rule: Thursday

On 5.2
RULE: entered rule: Thursday
RULE: entered rule: Wednsday

Who knows how this was fixed?

Regards
-W