Category ID Issues after Upgrade to 2.8.5

I have a theme that displays a unique header image for the archive page based on the category ID.

This was all working until I upgraded to 2.8.5. Now some category archives work, but there are several that all display the same category ID.

For category 4: <h2 class=”pagetitle cat29″> <= wrong
For category 6: <h2 class=”pagetitle cat6″> <= wrong
For category 29: <h2 class=”pagetitle cat29″> <= correct
For category 31: <h2 class=”pagetitle cat31″> <= correct
For category 8: <h2 class=”pagetitle cat31″> <= wrong
For category 1: <h2 class=”pagetitle cat31″> <= wrong
For category 5: <h2 class=”pagetitle cat31″> <= wrong
For category 7: <h2 class=”pagetitle cat31″> <= wrong
For category 30: <h2 class=”pagetitle cat31″> <= wrong

The code for the title is this:
<h2 class="pagetitle cat<?php the_category_ID(); ?>"><span>Archive for the ‘<?php echo single_cat_title(); ?>’ Category</span></h2>

Why would this change and how can I fix it?