Is this a bug ?

An issue seems to recently have surfaced in a visual force application developed by our organization. The issue pertains to table headers in a datatable where each iteration contains columns with a rowspan attribute greater than 1. In the datatable, table headers are being output for every column in the iteration, even for columns after the "breakbefore=true". This only surfaced in the past few weeks.

Below is an example of a simple visual force page I've created to illustrate the issue. In the example, there's a datatable where you would expect only 3 th tags to be output. Tthere are actually 5 th tags being output which adversely affects the layout of the table. In our affected application, there are 13 columns output which span 3 rows which means there 26 additional unneeded th tags being output.

<apex:page standardController="Account">
<apex:pageBlock >
<apex:pageBlockSection columns="1">
<apex:dataTable var="l" value="{!Account.Equipment__r}" styleClass="list" width="100%">
<apex:column value="{!l.Id}" headerValue="Id" rowspan="2" />
<apex:column value="{!l.Name}" headerValue="Name" />
<apex:column value="{!l.unit_desc__c}" headerValue="Desc" />
<apex:column value="{!l.supp_id__r.Name}" breakbefore="true"/>
<apex:column value="{!l.orig_nbv_amt__c}" />
</apex:dataTable>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:page>