Update: Solution/fix has been provided here.
This is mostly for WP theme authors with online demos updated to WP 4.4, which might end up looking funny
Via WP Forums
The 4.4 update breaks the display of the Tag Cloud widget if arguments are passed through the ‘widget_tag_cloud_args’ filter. I, for one, passed a smaller tag ‘number’ and a different ‘orderby’ and the result is basically a simple list of various font-size links, instead of displaying the defined widget format with surrounding tags and widget title and so on.
Removing/commenting the custom arguments filter solves the issue for now and hopefully the next (quick) update will resolve this issue in full so that we can still use custom arguments.
Hope this helps other authors. If anyone knows and shares a workaround, some way of passing those custom arguments and not break the widget’s formatting, it would be great.