visualforce controller for loop

I have struggles with a certain part of my visualforce controller. As far as I know there is no reason why after a loop execution should cause the data to be nilled or made inaccessible. But that's exactly what happens as far as I can see. below is the code snippet which iterates over a custom date format (yyyyMM) as Integer and within over a list of Opportunities to compare the dates.

opps = [ SELECT Close_Date_Month__c, CloseDateYear__c, Account.Name, CloseDate, RenewalDate__c, Name FROM Opportunity
              WHERE RenewalDate__c >= :dateFrom AND CloseDate <= :dateTo AND (StageName LIKE '%yes%' OR StageName LIKE '%Closed%') AND Account.Type LIKE '%Manufacturer%' ];
map<String, map<Integer, set<String>>> aMap = new map<String, map<Integer, set<String>>>();
for ( Integer i = getMonthYear(dateFrom); i <= getMonthYear(dateTo); i = getNextMonthYear(i) )
{
   ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.WARNING, 'YearMonth processed: ' + i));
   for ( Opportunity o : opps )
   {
      ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.WARNING, 'Opp processed: ' + o.Name));
      if ( getMonthYear(o.CloseDate) <= i && getMonthYear(o.RenewalDate__c) >= i )
      {
         ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.WARNING, 'Close Date: ' + getMonthYear(o.CloseDate)));
         ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.WARNING, 'Renewal Date: ' + getMonthYear(o.RenewalDate__c)));
         String s = getStringDate(i);
// Logic
       }
    }
}

I filled the code with ApexPage Message Warnings so as to see what happens. Here is the result:

Here are my functions used to get the formats:

// Extracts the year out of the integer of format yyyyMM
private static Integer getYear(Integer i)
{
    return (Integer)i/100;
}
// Transforms a date into an integer representation of yyyyMM
private static Integer getMonthYear(Date d)
{
    return d.Year()*100 + d.Month();
}
// Returns the next month of the Integer representation yyyyMM
private static Integer getNextMonthYear(Integer i)
{
    return Math.mod(i, 100) == 12 ? i + 89 : i + 1;
}
// Returns a 3-character representation of a date Integer yyyyMM
private static String getStringDate(Integer i)
{
    map<Integer, String> mapping = new map<Integer, String>
    {
        1 => 'JAN',
        2 => 'FEB',
        3 => 'MAR',
        4 => 'APR',
        5 => 'MAY',
        6 => 'JUN',
        7 => 'JUL',
        8 => 'AUG',
        9 => 'SEP',
       10 => 'OCT',
       11 => 'NOV',
       12 => 'DEC'
    };
    System.assert(Math.mod(i, 100) >= 1 && Math.mod(i, 100) <= 12);
    return mapping.get(Math.mod(i, 100));
}