Amount value comparison rule produces inconsistent results - Drools

Question

I inherited a program that uses drools api to run SettlementMessage objects against drl rules files defined. This program was running consistently for about 8 years in production. We now have been asked to move from Solaris SunOS to IBM AIX machines and started seeing some inconsistent results with few rules that compare amount field to a value.

public class SettlementMessage implements Serializable {

    private double Amount=0;

    public double getAmount() {
        return Amount;
    }

    public void setAmount(double amount) {
        Amount = amount;
    }
.....

}

Rules that are producing inconsistent results are defined as follows:

rule "Settlement Rule 1.1 - Large Settlement Amount For JPY and HUF - External Group"

    when
        $message: SettlementMessage(amount > 100000000.00, currencyCode in ("JPY","HUF"), settlementMethod in ("DIRECT_DEBIT","SWIFT", "SWIFT_EREL"))
    then 
        $message.setBusinessException("Exceeds Threshold - Confirm Settlement.");   
end


rule "Settlement Rule 1.2 - Large Settlement Amount For Others - External Group"

    when
        $message: SettlementMessage(amount > 3000000.00, currencyCode not in ("JPY","HUF"), settlementMethod in ("DIRECT_DEBIT","SWIFT", "SWIFT_EREL"))
    then 
        $message.setBusinessException("Exceeds Threshold - Confirm Settlement.");
end

There are lots of other rules that are matching different attributes of SettlementMessage object against some values and all are working as expected. We only see issues with these rules matching objects with amount well below defined threshold.

We process large number of SettlementMessage objects through these rules each day. Normally when system date advances - we would process between 500 and 1500 SettlementMessage objects. Objects are processed in sequential order, not in parallel. On most days, most objects would get processed correctly - if amount is below specified threshold then rule is not matched. If amount is above specified threshold then rule is matched. On some days, some objects would incorrectly get matched against these rules (amount is below value threshold defined in the rule). We see examples of SettlementMessage objects with amount anywhere between 1 and 3000000 get matched against one of the rule (depending on currency value). Java program that creates KieSession and runs rules is packaged as jar file and is called from Tibco Business Works adapter. KieSession object is created as PooledObject and is reused between multiple calls for each SettlementMessage object. KieSession object lives as PooledObject as long as BW adapeter is running. SettlementMessage object is inserted, fireAllRules is called and SettlementMessage object is deleted.

Some interesting observations: When we get objects that were matched incorrectly against these rules, if I resend same (clone) SettlementMessage object again, it will match incorrectly this rule again. If I restart BW adapter and resend same object again, then it processes correctly. It's like KieSession or underlying decision tables get somehow corrupted in memory, but after restart everything goes back to normal. Original application was using Drools 5.0.1 API and I rewritten it to use Drools 7.12.0 instead and this had no impact on behavior.

Does anyone has any idea why this amount comparison would mismatch on some objects on some random days? Is there a way to rework this rule to accomplish same intended logic, but consistent results (no mismatch)? I tried changing the syntax of these rules a little bit, but had no luck fixing my issue. The biggest problem is - for every change that I want to try, I can't reproduce this issue on my DEV machine and I need to wait for days on IBM AIX machine to have it reoccur.

SunOS machines were running Oracle Java.

IBM AIX machines are running Java IBM J9 VM 2.9.

I would appreciate any insight into how to rework this rule that may have some impact on the issue I'm facing.